Homebrew PCBs

Is it necessary to expose both sides of a photosensitive PCB 
simutaneously, so that I can dip the entire board into a sodium 
carbonate tank?

Otherwise if I expose one side first, I have to develop it 
immediately, otherwise it will be overexposed.  However it would not 
be nice when I have to dip the entire board (include the opposite 
side which is covered by tape and not yet exposed) into the 
developing solution.  Also, it would be messy if I spray the 
developing solution onto the exposed side because the developing 
solution will still seep onto the unexposed side!

Mark