My husband and I did this several times with two...the best thing to do is stop often. If she is normally in an infant seat but is big enough for the weight requirement for the up right and she is not bottle fed anymore then go ahead and get an upright. If she is still taking a bottle then leave her in the infant seat or plan to stop to give her the bottle. We had one in an infant seat and one in a car seat. When it was time to eat she got her bottle with no problem...I slipped in the back seat. When we traveled with just one (while pregnant with the other) I sat in the backseat the whole time. That way I could entertain her and feed her and not have to climb over the seat often. Do plan to stop so she can be changed and stretch. Just like you she'll, get tired of sitting. So every couple of hours plan to stop for 15-30 minutes. Make sure you have her favorite toys to play with and a favorite blanket. If you have a travel DVD player, take it and pop in some Baby Einstein or cartoon movies. I wish those were around when mine were little. The colors and movement will keep her occupied for a while, since she doesn't have much to look at. Don't stress so much, she'll sense it and the ride won't be fun. Good Luck and God Bless.