You take a quick peek down each passage.  

The main passage is ten feet wide. It's a short tunnel that ends at a slab of worked stone, whose edges are marked by relief carvings of grinning skulls. Four lines engraved at the center of the slab cross one another to form a star, with both ends of each line marking the location of a cube-shaped cavity cut into the door- eight cavities in all.  (aka, eight empty squares in the large door, with a line connecting pairs of them.)

The smaller passage to the east runs twenty feet into darkness. Beyond that, alcoves run the length of the narrow tunnel, each one featuring a bestial statue standing or squatting above a basin of oil.  From here, you can see that moving down the hall, the opposing statues depict Moa (jaculi) and Wongo (su-monster), I'jin {almiraj) and Obo'laka (zorbo), Papazotl (eblis) and Kubazan (froghemoth), and Nang- nang (grung) and Shagambi (kamadan).  The last trickster god is not represented here.  Unkh (flail snail). 

All of the trickster gods represent real creatures that are native to Chult.

Moa is a Jaculi, which is a fifteen foot long snakes that can alter their color and appearance.

Wongo is a Su-Monster, an evil ape (you met some of these in the jungle).  According to the story, Moa and Wongo are enemies.

I'jin is an almaraj, a small bunny rabbit with a single horn in it's forehead.

Obo'laka is a Zorbo, a small orange colored bear like creature that is ferocious. According to the story, I'jin and Obo'laka are enemies.

Papazotl is an Eblis, the crane like creatures you met at the gardens.

Kubazan is a froghemeth, a massive frog creature you've never met, the size of an elephant. According to the story, Papazotl and Kubazan are enemies.

Nang-nang is a Grung, the small frog people you've met and fought. 

Shagambi is a Kamadan, a leopard with six snakes growing out of its back. According to the story, Nang-nang and Shagambi are enemies. 

Unkh is a flail snail, a large snail with a scintillating shell that can daze creature. It is also a fierce combatant with mace like tentacles. 

You've just glanced down each hallway and not entered either or examined them in detail.  Please ensure you tell me who goes first.  (OOC: NO REASON!!!!)   (FYI, Generally as a DM, I tend to ask things like this, even when it doesn't matter, just to keep my players honest....though I can't say if this is such a time...) 

"Hmph, well this is the most unpleasant riddle I've ever seen. I suppose if Unkh is the creature from the legends that stopped the fighting, and the riddle seems to indicate that the 'One who stands between' is important, we should seek that creature out? Perhaps we can use its shell to create some sort of protective mask like in the riddle." Mandra looks down the hallways and at the glyphs on the walls. "Does anyone see any unmarked doors or anything that would lead to Unkh?" She'll spend a few minutes investigating the main platform for any secret switches or doors. Investigation: 10

Mandra examines the threshold of each hallway carefully but doesn't spot any obvious switches or doors. 

If he thinks he can do so without attracting the attention of the gargoyles, Sickle lights up a hand with a Produce Flame and looks down the hall with the opposing statues.

Also, is it obvious if the statues would or would not fit into the grid of 8 down the other hallway?

Casting Shield of faith on himself, Silas will stand as the front runner, armed with his shield and warhammer, into the tunnel, "Come on, no use standing around if there's something to see," He marches into tunnel with the status. If he can, he will cast light on his warhammer as a makeshift torch as well. 

Silas marches down the hallway, his torch lighting the way. The statues are not cubes, and too large to fit in the door anyway. Each one is hung by chains over vats of liquid that appears to be oil. The statues on each side of the hallway are paired into the trickster gods that oppose each other.  Nothing else obvious appears present in the hallway. 

Badger will investigate the grid to see if he can determine what manner of mechanism it is, and what kind of objects it requires. 23

(Trying to post on my phone in a hurry)

Fail?

Badger wanders into the wider, shorter tunnels that ends in the door with 8 empty square spots in it. They appear to hold a cube sized objects, 3 inches square. But he sees no physical mechanism that putting cubes into the spots would trigger. 

Mandra puts a hand on Silas's arm, "This oil looks flammable." Arcane flame tickle the ends of her fingertips. "Let's stand back a bit."  She stands as far back as she can while still having visual on the last bowl of oil before casting Fire Bolt on the oil.

The wide tunnel is 10 feet wide, 15 long, and ends in the door with the 8 cube spots in it. The narrow tunnel is 5 feet wide, 40 feet long, has 8 statues in alcoves along the edges and has no doors. 

The fire strikes the oil which, for a moment appears as if it will light up, but then the flame goes blue then goes out.  

"As you can see, this oil is not normal, I think it has something to do with these trickster gods?" Silas question as he shines his warhammer light closer to each statues before looking around for the missing one, "Perhaps the missing one is the key to this location?" 

Perpcetion: 19

”I thought the same. Hmm. Perhaps these statues descend into the oil somehow.  Also, the story mentioned the snail coming from the ground. Perhaps somehow the other statue is in the earth?”

Sickle will study the chains to see if there is anything that may be learned, and also pay special attention to the ground.

Perception: 21

Mandra nods in agreement to Silas's comment, "The rhyme does seem to indicate the snail Trickster as being the key to this." 

She will also try setting two opposing bowls of fire at the same time with two Fire Bolts in quick succession. "Maybe more is better?"

Sickle will assist with a Produce Flame shot at an opposing oil vat.

Silas searches around but doesn’t find any hidden doors or mechanisms. Sickle, there is no obvious mechanical mechanism for the chains. They look mundane and attached to the stone.

The group flings more magical fire at opposing vats with the same result. 

"This may be a place we return to but we can not move forward in this place," Silas states looking at the statues before turning and ushering everyone behind him out. 

Badger turns away from the grid, rubs his nose, and says: Urgh, I expect there are eight small cubes that when inserted into this grid affect those statues. Perhaps if we chance upon them we can explore this further.

Sorry, I read this a 2nd time, and you said "explore THIS further", not "explore further".  Do you guys continue to examine the two tunnels, the obelisk, or something else?  Or do you head out into the city to continue exploring?