Can a late 2013 Mac Pro (cylinder) support dual 5K displays? I know the late 2013 Mac Pro can support the Dell 5K monitor, using two DisplayPort cables from the Mac to the monitor.

Can the Mac Pro support two 5K monitors simultaneously (using four cables total)?
Yes - three 5K displays are supported on even the least GPU endowed Mac Pro Late 2013.

While I do not have the luxury of owning this kind of a setup, the following coming directly from Apple should answer this:

Mac Pro (Late 2013) - Technical Specifications