Actually Even mining boards with a single long slot work fine. i have done this on H81 and even TB85. 1 4gb in gpu 0 slot mining eth. The other gpus on another coin on risers. Running the one card on eth pays more than running it on any other coin currently. The other slower cards are better on other coins than the reduced speed due to dag on eth.
what linux os did you used? Did you be able to run eth + rvn on same machine? could you pelase explain more?
For dual mining or triple mining or even quad mining i run mmpos (mmpos.eu)
For some dual mining i run ethos (which is very old but still works well on older cards
For two rigs they are just ubuntu current version. Dual mining is as easy as using the screen command to have several screen to run from. Google screen command in linux for syntax and usage.
I use the -devices= option in lolminer and teamredminer to select the cards for each miner regardless of linux version used.