At present, the dual GPUs on the 2013 Mac Pro have extremely limited support in software applications (this is actually true of all Macs of all years, but the 2013 Mac Pro holds huge promise in particular).
A certain software application vendor contacted me very recently, asking if I were interesting in testing a beta version of software that will utilize both GPUs on the 2013 Mac Pro. Naturally I said yes, and I hope I’ll be able to report on the results soon, the exciting part being that optimized use of both GPUs could deliver 3X to 7X the performance one gets from CPU-based processing. That’s particularly exciting for photography (video too, but it has fairly good GPU support already).
Will it be a sockdolager performance upgrade as for the value of GPU computing? One hopes so.
In the meantime, see 2013 Mac Pro GPU Benefits to get a sense of half-way implementations that don’t put both GPUs to work.