I love the site but something bugs me when homebrew subclasses come into play. I tried to create a Rogue subclass that actually benefits using the classic duel daggers by giving Expertise with them. My issue is, quite simply, it doesn't translate into the character sheet. 

Daggers are seriously under powered in 5e and as they are a trademark of stealth and shady elimination, I wanted to rectify that. I've tried using Expertise and Double Proficiency simply to hit (daggers are for precision) but it doesn't work on the character sheet (still uses base proficiency and Dex bonus). 

Can anyone help?

Two points:

Not all modifiers subtypes work with all modifiers like that. All of the possible proficiencies are listed as modifier subtypes for the “proficiency” Modifier, so they also get listed with the other Modifiers that have to do with proficiency, but they don’t actually interact with them since nothing official grants either double proficiency or expertise in any weapons.

Daggers are not under powered for rogues because the actual damage die of the weapon is irrelevant when Sneak Attack damage is getting applied. Those multiple d6s are way more relevant than the dagger’s d4. What dual-wielding daggers does for a Rogue is give them a second opportunity to hit with that Sneak Attack damage in case they miss with the first attack. Rogues dual-wielding daggers often outperform Barbarians when it comes to DPR if the Barb isn’t running a Great Weapon Master build.

So the answer to my conundrum of "why can't you add double proficiency to a weapon type" is simply because nothing official allows it. Thank you for the reply.

As for Daggers being under powered, it's more of a precision issue. Looking back on history, soldiers and knights kept daggers on their person to deal killing blows to opponents in full plate as it was easier to deal a precise strike through gaps in their armour such as the visor slit. Mearly adding an Expertise in a weapon type would increase accuracy which is what I was trying to accomplish, as because of the lack of such, there's more sense duel wielding shortswords over daggers using what is official.

I do appreciate the input, though I don't agree with the premise.

Keep in mind that double-proficiency in weapons is effectively giving a level 1 Rogue a +2 weapon (if not +2 to damage), which is a Rare magic item. Any increase in To-Hit makes a very big difference in 5e combat.