You can't put 8 ohms into a 6ohm speaker, impedence is a property of the speaker and no switch you can flip can change that. On the other hand, asking an amp to drive speakers with too low an impedence can kill the amp, this is unlikely though as most have some kind of limiter in them (at the least a fuse!)
Either the amp can drive a 6ohm load using both it's speaker outputs or it can't, the spec's say it can't but you never know - you'll soon find out.
However, why not just single wire the speakers, it won't make any difference and you can be sure your amp will be happy.