yup you pretty much nailed it, for dualmode it has to be CGMiner-Dmaxl, if you want to run different miners, eg a gridseed on scrypt and a U2 on sha256, then as you did, run one in minera, the other in a seperate instance from the rc.local in settings page
I just tried to go dual mode with the Gridseed using the cgminer-dmaxl-zeus, while having the Antminers use BFGMiner. Everything got borked
I just went back to my original configuration of BFG for Ants in Minera (the web interface only detects the ants for some reason) and BFG via rc.local as described earlier.
I also tried just CPUminer, which set everything to 800Mhz and only the Gridseed worked for kh/s in BTC and regular CGminer, which only detected the Ants. I am hoping that I didn't fry anything, but good grief, you think this would have been easier to deal with. Hopefully Minera will eventually be able to handle multiple devices via the web interface.
As for the CGMiner-dmaxl-zeus, it seemed to grab the Ants and not the Gridseed, or it would go vice versa, but not both. Any advice?