Rope type when bringing 2 climbers up a multipitch

What is the current accepted rope type for bringing 2 climbers up a multipitch route. I've been using 8.4mm half ropes, red tied to one and blue tied to the other. I want to know if this is okay. If not, what is?

That's the way to do it.
Just remember to protect both climbers when traversing.

You can bring up two followers on half/twin ropes, but you will have more stretch on each rope if one of the climbers falls and you won't have the edge/cutting protection of a single.

I think it's optimal to use two single-rated ropes.

I always use half or twins when bringing up 2 seconds. Works well.

depends on what you have with you and what your are comfortable with. Could also isolate a 10ft bight on a single and have your weaker follower tie in there, and the stronger on the very bottom.

Its always interesting/annoying that no one will work skills out themselves, hurrr durrr innanets tell me how to live.

Yeah using half ropes is pretty common. A top-rope (that is following) fall won't put nearly as much force on a rope as a lead fall, so this is fine. It will be a bit more stretchy for the fall, if one of the 2nds falls, but not huge compared to, say, sling-shot TRing the same distance on a route.

You're fine with 8.4 with a second on each rope.
8mm ropes may get problematic with a weak second or two, I've noticed with the ATC guide and the Reverso 4 that when it's in guide mode, to "climber" rope can wedge itself on the side of the "brake hand" rope and lock it up if it gets loaded. you can release it but you have to be careful (keeping your hand on the brake end, etc.)

Petzl warns of this:
Warning, the REVERSO’s self-braking function may be disabled
1. Warning: when belaying one or two seconds on ropes less than or equal to 8.5 mm in
diameter, the ropes can cross and disable the self-braking function.
Always hold the brake side of the ropes.

Wished Petzl had kept the Reversino for dealing with the smaller ropes out there.

one important note ... twin ropes are not recommended for bringing up 2 seconds

now plenty of half ropes are rated as twins and those would theoretically be fine ... however use your judgement as while the rope wont break, if the second takes constantly or swings you could saw right through a thin rope