I've never made 2 half sheets side by side so...after flipping the first cake out how do you get the second one out next to it?? I usually put my cake board on top of the cake pan and flip but can't do that with the second one.
Freeze the cakes and then you can handle them easily. Or flip the second cake onto a smaller board or sheet pan, then slide it off into place on the big cake board.
Flip the second cake onto a cookie sheet (one that has no sides) and then you can slide/push it onto the cake board right next to the first cake. You have to remember to put icing on the side of the first cake and under where the second cake will go so that it will stay anchored to the first. HTH
I've never had to glue the pieces together with frosting. I just ice the thing like it's one cake and it works out fine.