What happens when Magus & Frog travel to a different era?

Since Magus was the one who turned Glenn into a Frog the curse appears to be tied to Magus' life. If you choose to kill him at one point in the game Frog will once again become human by the end of the game. But what happens when they both visit a different era? Like when Frog travels to the future and Magus stays behind... shouldn't that break the spell at least temporarilly? That would be logical since Magus and Frog are technically not living at the same time. You could argue that they are tied by a spell throughout time. In that case however, it wouldn't make sense that Frog becomes human if Magus dies, because Magus would still be alive in the past.