Can't help much with using spey lines for non-spey casting. Be aware that spey lines are totally different than regular lines. If your rod is rated as a 7 wgt, then a 7 wgt spey line will overline it by quite a bit. As a for instance, for a 4 wgt spey rod, a 7 wgt regular line is suggested just to get it to load properly.

Instead, try using spey casts with your single hander and regular line to get your big flies to turn over.