If my pump curves (UltraFlow pre-94) don't show results for the low speed of a two-speed pump, does that imply that it won't work (or at least not well), or just that two-speed motors weren't common back then (1991)?

I understand that I will have to size the motor correctly. But I'm wondering whether there was something about the impeller design that precludes the use of a two-speed motor (running at the lower speed). Thanks.