All I can tell you is that the times I've flown GUA-FRS-GUA, it's been fine and things have been reasonably on time. But all it would take would be a mechanical or weather delay or cancellation and, poof, there go your plans. No one can predict that. Personally, I would never risk this on separate PNRs. I tend to be overly cautious. I would fly back and spend the night before in Guatemala City.