Depending on what algorithms you want them to run on, you would do it differently. I don't know how to use a gridseed on SHA-256 though, maybe someone else can help.
You would install the drivers, plug in the gridseed to your computer, and run a cgminer (3.7.2 is the latest that supports scrypt) and run another instance of cgminer/bfgminer to mine with the block erupters on SHA-256. (I'm assuming they are Block Erupter USBs?)