I'd say the friendship is 60% husband, 40% wife.
Does your gf have the same mathematical overview of the situation girls sometimes like to stick together, you know moral support for a friend in need
If you go and they start some static is it going to be boys go one way and girls go the other to chat and socialize ???
If you go in separate cars and they start some static then you could end up stuck with one of them and which ever one it is will probably bring your enjoyment on the trip down???
This vacation picture your painting has higher percentage chance of failure than good times and success. I'd say 70% chance of bad and 30% chance of good.. I don't like to gamble but I wouldn't take those odds
Whole point of the vacation is to try and leave drama and stress at home, sounds like you packing 2 suitcases of it and taking it with you....
goodluck