I'm of the same opinion as most here- I encourage my wife to indulge herself in whatever kooky solo fantasies she has- but if she refused to let me join in, it would upset me. Granted, I wouldn't ask if it would spoil the moment, and I'd be just as likely to let her watch me, ya know, if it helps.
If she became too preoccupied with the solo act and it affected the quality/enjoyment of the together act, that would be problematic as well. (I think this is the problem with many guys masturbating- too into
www.hotbabesinheat.com, they forget about the ACTUAL hot babe in heat in their bed.)