Just wondering if you play a lunar sorcerer and cast sacred flame, can you twin the spell using metamagic? 

The spell targets 1 person so technically it can be twinned, but the level 1 ability Moon fire allows you to target a 2nd which seems to invalidate twinned spell.

And if you can does that mean you could target 3 or 4 creatures with sacred flame?

I'd imagine no. My understanding is that even indirect effects cannot hit a second creature. I tried to argue once that dragon breath for example, the spell only effects one creature, the breath is just an extra action given to that creature and not the direct spell. But because you can breath multiple people with it , dragon breath can't be twinspelled. I'd say it's the same principle here. 

You can only target a second creature if it is within 5 feet of the original target using the level 1 feature so you would not be able to twin that - nor would you want to burn a sorcery point if you don't have to. Target the spell as normal with a 60 foot range and you would be able to twin it. 

The class feature gives you a variant of Sacred Flame with multi-target ability.  Therefore it's ineligible for twinning the same way that Acid Splash is.

I can see those interpretations, but...

Moon Fire:

You learn the sacred flame spell, which doesn't count against the number of sorcerer cantrips that you know. When you cast the spell, you can target one creature as normal or target two creatures within range that are within 5 feet of each other.

The use of the spell to me, indicates that you're casting the normal spell, but it's modified after casting to target the 2nd creature.  By RAW if not RAI

That being said I can completely understand any DM giving a big 'H E double hockey sticks NO!' to that interpretation. 

I just think it would be cool, and not too game breaking to allow 3-4 targets using a Sorcery point.  

Another possible argument is using Lunar Bonus (6th) on Sacred Flame it costs nothing if you're in the New Moon Phase which means that the Moon Fire ability is a waste when you can twin the regular form and get better range, so basically invalidating the 1st level ability.  I know the 1st level abilities are generally not used later in the game but for it to be basically obsolete by 6th level seems a bit much, so I wonder if it was intended to do this or an unforeseen side effect.  Also would 3 targets for Sacred Flame (or even 4) be that game breaking?

Relevant text from Twin Spell:

To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.

As Moon Fire lets Sacred Flame target more than one creature, it can't be used with Twin Spell.

Quote from 6thLyranGuard >>

Relevant text from Twin Spell:

To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.

As Moon Fire lets Sacred Flame target more than one creature, it can't be used with Twin Spell.

exactly this

6thLyranGuard is correct in RAW, though it's less clear if this is intended, because technically it also means you can't twin it even when you want to cast it at two targets further than 5 feet apart (i.e- twin cast the ordinary, unaugmented [Tooltip Not Found]) because even though you won't get to pick two per cantrip, you still could.

Personally I'd allow single target flame to still be twinned, but definitely no twinning the double target flame, as you could theoretically just hit the same two targets twice each (choose creature A as your initial target, and creature B as the bonus target, then for the twin cast choose B as the initial target and A as the bonus target), which would be a lot of mileage out of a cantrip for just one sorcery point.