I am in the same boat. I currently have 2 FX5's on a standard 100g. Right now I have it crossed. Intakes on the outside and outputs on the inside. With this setup, I have noticed great circulation with no need for surface agitation. That being said, because I have "slacked" on cleaning the FX5's (over 6 months), that they are struggling on picking up the large poops my umbees leave.
I was going to clean them out soon and keep the setup the way it is, but am leaning towards making it a one-way turbulation. Both intakes on one side and both outputs on the opposite end.
I think its personal preference though, either way. Whats the stock list? And are you going for habitat simulation or maximum tank circulation?