I think it is risky trying to get a sheet of veneer to conform with double curvature. I have not tried it, but it might well be asking too much of the wood, however pliable you make it. If it was me, I would plank in both directions, then you know you are going to get a properly shaped deck.
Of course, if you do that you will probably have to install some deck beams/stringers to support the first layer of planks, if they are not there already.
Greg