The 990's balanced and unbalanced pre-outs are both active, if I remember correctly, so this is a possible solution assuming that the 997 behaves similarly. You would need to power amps on/off manually to control which zone was active, and if you wanted to use both at once you'd have to live with the same source, processing mode, and volume setting for both. Also, the 755 only has unbalanced RCA connections, so you'd need to add an amp with XLR's (or use some XLR-to-RCA adapters) in order to make this work - which may not be a problem since you need a second amp in this scenario anyway...