Hello K.,
My boys are 8 and 3 so I don't have the problem of 2 in seats at the same time. What I did was kept my older boy in the booster until he was too big for it, but that was at 7 years old. My boys stayed in the forward facing car seat until they outgrew the capacity for it, and then went straight to the Graco booster that has a back to it and cup holders to either side. the back part adjusts up as they get taller to keep the lap belt going across where it should. My older boy grew out of the booster before my younger one was ready for it. My 3 year old is 3 foot 4 inches tall and 45 pounds, so he is big enough for the booster anyhow. He stayed in the forward facing car seat until he was 3 though. I would do it that way unless your 2 year old is already at the capacity for the seat he is in.
I also take the 12 hour drive to CA, only we take it 2 times a year to see family as I have no family here. Where in CA do you go?? I go to Gilroy, CA.
D. P.