Dual-monitor setup: one with g-sync, one without?

Hello guys.

I'm thinking of buying a second monitor. I currently have a Dell P2416D (no adaptive sync tech), but I play a decent amount of games and so would want a higher refresh rate coupled, ideally, with G-Sync.

I've heard that switching from one monitor with G-Sync to one without can make things act a bit weird? Is this true?

Bonus question: How would running in SLI affect this situation?

I'd very much appreciate help. Many thanks in advance.