Yep, do it all the time.
You don't need to setup Crossfire or SLI for mining purposes and running multiple cards on a single motherboard won't be an issue.
In fact I've seen the extra compute overhead from Crossfire and SLI actually slow mining down, it's probably better to NOT use those features.
The only thing is that you may need dummy plugs for the cards not hooked to a monitor if you're using a Windows OS.
This.
I'm getting into mining myself and from everyone I hear SLI pretty much poops on your hashes. I'd say get some PCI extenders and call it a day.