I'm sure there's more than 1 way to skin this cat but yes,  my suggestion was to call 2 instances of the executable within your batch file. The important aspect in those calls however is the -di parameter which specifies the GPUs to be allocated for that instance.

My response to lazyturtle is applicable here as well - just substitute the second/solo mine call for whichever pool you need for the 2GB cards.

For example:  
EthDcrMiner64.exe -epool eth.coinmine.pl:4000 -ewal lazyturtle.1 -epsw x -di 012 -dpool sia.suprnova.cc:7777 -dwal lazyturtle-dpsw x -dcoin sia -allpools 1
EthDcrMiner64.exe -epool <EXP.pool.here> -ewal lazyturtle.1 -epsw x -di 345

Yes.  You would setup 2 separate calls to EthDcrMiner64.exe, each setting the -di parameter to indicate which GPUs to allocate to that particular mining task.

For example: 
EthDcrMiner64.exe -epool eth.coinmine.pl:4000 -ewal lazyturtle.1 -epsw x -di 012 -dpool sia.suprnova.cc:7777 -dwal lazyturtle-dpsw x -dcoin sia -allpools 1
EthDcrMiner64.exe -epool eth.coinmine.pl:4000 -ewal lazyturtle.1 -epsw x -di 345

Has anyone else's hashrate essentially halved using RX480?  I was never getting world-class hashrates (~23MH/s) but now it's down around 13MH/s for the two RX 480s that I'm testing. [ETH/ETC mining]  For reference,  I'm dual-mining SiaCoin and that halved as well from 7xx to 3xx.

Disclaimer:  I did install an RX 570 into the system to test,  didn't get the results that I was after so disconnected the card and uninstalled the AMD drivers completely before reinstalling 16.9.2. 

Suggestions?