Spend another $150 and get another motherboard, anything that supports three - four GPU's will be fine, a cheap Celeron processor, some memory, and a SSD. You don't need a case, look around the forums for examples of some open air rig setups, or even milk crate will do. You already have two PSU's, one for each rig. You will be setup and running a lot quicker and less grief overall.
I have used the H81 Pro BTC before, and while you can eventually get it to work, it is not as stable as breaking that equipment up into two rigs. You will be much happier in the long run. In the future go with 4 GPU builds and a 1250 watt PSU, most bang for the buck and less grief.
No one serious about mining should listen to any of that. If you are new, its the easy way out. Otherwise build 6 or 7 card rigs....the right way. Linux, H81, or H97 6 slot Motherboard work fine, with powered risers and motherboard running on the same PSU, use the second PSU to only power the 12V VGA connectors for three cards.
It's way cheaper than running $150+ for another motherboard, CPU and RAM to split them up.
BAD ADVICE!