yes, a light blanket or towel for the trip is a great idea in case of accident. bring two extras so you can switch them out if necessary. mine never eat int eh carier, they’re way too stressed, but it doesn’t hurt to bring along some hay like Sarita suggested. make sure to feed them normally up to the trip and have food ready for when they return. initally when i was reading your post i was going to say if they’re bonded to put them in the same carrier and they’ll comfort each other, but i forgot they ARE NOT FIXED!!! so definitely you need two carriers! lol.
something to think about for the future, can they both fit in one carrier? if they can’t comfortable both fit in one carrier for once they are bonded, i would take back one carrier today and get the next size up. i have two carriers, one small, one medium, one is for single bunny trips, one is for dual bunny trips. this way you’re not wasting money on something you won’t be able to use later.