Make sure you have a strong board to support the cakes evenly. I usually get the thick foam board and cut it to the size I need to fit the cake and box. So I do not have any gap between the two cakes I set one down first and cut the long edge so I take any outside crust off and then lay the second cake on the board and do the same. I put icing down on the board under where both cakes will be so it will hold it them in place, just a swipe of icing with your knife is enough. I then ice with a little more than a crumb coat on the cut edge and slide the other cake right up to it. Then you can just gently push it to the first cake so they join almost seamlessly. I also mak sure the cakes are chilled so handling them is easier if they are large than 9x13. I hope this has helped you and good luck with the cake! 
Let us see a picture when you get done.
evelyn
Thanks for the help! I've been making my cake boards out of foam core. I use three of the thinner ones spray-glued together...which looks to be about an inch thick. Maybe I'll add one more board just to be sure.
Thanks again for the help...I feel much better 
Cake isn't due until next Saturday....I'll make sure to post then.
I use mdf for full sheet cakes because cardboard just isn't strong enough.
Your cake looks great!!! I hope that all wisdom that was imparted to you helped you out. 
In other words, we can say we were glad to help.
evelyn
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