I don't think having two ultras at once is going to change much for most of the cast. For example, VS Ryu - many characters would just stick to their anti-projectile Ultras anyways to punish Ryu really hard for a poor fireball.
I don't know how much having both ultras will lower the damage by, but if they lower it by 30%, that means Zangief's U1 will do 357 maximum instead of 510 and U2 will do 315 instead of 450 max. In that case, I'd rather just pick U2 and use my regular ground grabs instead.
The only characters I see benefiting from both ultras are Makoto, since both will give her some much needed (but still risky) defensive options, along with Fei Long + Cammy who can intimidate people with a locked and loaded U2 while still being able to add damage off a Dragon Kick FADC.