Ok so I'm a little behind on this since I know it's been discussed in the past but I have so many questions! Looks like we'll be traveling down to Florida next April and we're taking both kids.
1. Do rental cars tend to have carseats or do we have to lug them with us? If so, that doesn't sound like much fun.
2. If you have 2 kids, did they share a room? If so, how did it go? Any tips? K will be 10 months at that time.
3. Did you just sit the kids on the plane in the seat or in their car seat? I don't think I've ever seen kids in a car seat on a plane but I've heard of people doing that.
That's all the questions for now, i'm sure i'll have TONS more when it gets closer. I'm just trying to wrap my head around it.

Any other tips or suggestions would be appreciated! Thanks!
Re: Flying/Vacationing with the kids
We just traveled last month and took the carseat with us. It was a PITA but the rental car didn't have one. I don't know if they provide them, but it probably would have been extra. I just have Ava sit in her the seat on the plane, she does fine.
DD 12.2010
I just flew with both kids and here is what we did.
1. My step-dad rented a minivan through Avis and they had car seats. I believe it was $50 deposit for each seat, but you got the money back if you returned the seat. I didn't want to lug 2 convertible car seats through the airport! Avis had convertible seats and I just installed 1 FF in the back for DS1 and one RFing in the middle/captains chair for DS2.
2. I shared a hotel room with both the boys and we had an adjoining room with my parents. I would put DS2 down in the PNP (13 months at the time) while DS1 was hanging out with my parents in their room. Then about 30 mins later I would bring in DS1 and he knew he had to be very quiet. We would lay on the queen bed together and I would quietly sing/talk to him and stay with him until he feel asleep. Once he was asleep I would sneak into the other queen bed and sleep there. I would usually just play on my phone for a bit and then go to sleep myself. They both STTN, but probably woke a bit earlier than normal and went to bed a bit later than normal. DS2 took naps in the hotel PNP, but DS1 often didn't nap.
3. DS1 has his own seat on the plane and just sat in the regular seat and did great. The flights were only about 1.5 hours and he pretty much played on the ipad or snacked the whole time. DS2 was a lap child and having a window seat really helped. He loved looking outside. He actually fell asleep on the plane ride home, which was awesome. I brought his lovey with us so he would have something comforting/familiar on the plane.
1. Do rental cars tend to have carseats or do we have to lug them with us? If so, that doesn't sound like much fun.
Some rental car companies offer rental seats I personally would never use a rented car seat. You don't know the history of the seat not to mention who knows if they wash them after each rental-yuck
2. If you have 2 kids, did they share a room? If so, how did it go? Any tips? K will be 10 months at that time.
Can't help you here we currently only have one child.
3. Did you just sit the kids on the plane in the seat or in their car seat? I don't think I've ever seen kids in a car seat on a plane but I've heard of people doing that.
My son as always just sat on the plane with out a car seat. My SIL has used the car seat on a plane before, she has a VERY active toddler and having her strapped in her seat was a huge help. It also makes sleeping more comfortable for your kids. If you are taking car seats anyway for your rental car you might consider taking them on the plane. Any child under the age of 2 will NOT need a purchased plane ticket you would just hold that child on your lap. You will however, have to let them know you will have an "infant in lap" either when you purchase your plane tickets or at check in. There are certain seats on the plane that you can not sit in with a lap child due to not having enough oxygen masks in that row. If you choose to check your car seats as luggage I'm pretty sure they do not charge you for that but you will need to check with the airline to be sure. There are also car seat travel bags that make lugging them around much easier, some even have wheels. They also protect your straps and buckles while on the plane and baggage claim.
2. We just got back from a week long vacation with both kids. ds2 is 1 now. it was interesting but like pp said, if I could put ds2 down first then A understood he had to be quiet and not wake little brother. We also had later bedtimes and some earlier wakeups, but ds2 doesn't STTN still and even when he woke it didn't wake up A and A is a super light sleeper most of the time.