I have problems with my BC cracking when I move it(sheet cakes). I have doubled my boards but still I have cracking. I use the wilton BC recipe with a little butter added for flavor. If you have any suggestions please let me know. I'm even willing to try a new BC recipe.
I've heard that you can add a little vinegar and that helps the cracking (small amt. - like 1 tsp per batch). There is so much sugar that you won't taste it.
But I think that the issue is still your board. Even a double board is sometimes not enough support for a sheet cake. Any give at all can cause cracks. I would try foam core for your boards or even masonite - something more sturdy than just cardboard.
I had the same problem and tried using vinegar and it did help slightly, but what i found most helpful was using a heavy duty board. I generally use a regular cakeboard and then place the cakeboard on a white cutting board for that extra support. I carry just about every cake on that cutting board and that keeps my bc from cracking.
Try the 1/2" foam core.