Would two AC70's create too much flow in a 55 gallon tank for a school of neon tetras? I'd like to have a very "serene" setting and am not sure if the flow would be too harsh for that kind of setup....especially for small fish like neons.

It'd be just fine.

I currently have 20 neons in my 55 gallon right now with 2 Aquaclear 70 filters and they're doing just fine. Also remember, the flow on the AC70 filter is adjustable.