How do I derate for 2 dryer circuits in one conduit?
Your neutrals don't count
NEC 310.15(B)(5)(a) governs this situation:

(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).

You'll be fine with the 10AWG
This brings you down to 4 current carrying wires, so you will be using an 80% derate off the 90°C column in the ampacity table as you mention in your question.  10AWG copper can carry 40A at 90°C, and 80% of 40A is 32A, which is greater than the 30A you're asking it to carry (and the 30A maximum it's allowed to carry by 240.4(D) small conductor rules), so you'll be fine.
Conduit fill will be fine too
You are using 14mm2 per 10AWG wire, and 6 wires, so 84mm2 in total.  This is well below the 137mm2 usable fill in a 3/4&quot; EMT, so you're fine on that front as well.
And continuous loads aren't a problem either.
220.54 allows one to use a 5kVA assumed load for dryers that do not have a larger nameplate load rating -- given that 24A at 240V is 5.76kVA, you should be fine on the continuous-load-derate front for most clothes dryers.