If it's a connection, yes. However, the "Perk" (yes, it's a stupid name. ) allows you more flexibility than that. With my example, you could book EWR-LHR, stay for a few days, then go to FRA, stay a few days more, and then return to EWR. (There are other, more creative things you can do as well, but this is one of the simplest). It is intended as a replacement for the stopover that United previously allowed on award flights.

If your intent is a simple round-trip, with or without connecting flights, you can safely split it into two awards as you've described. But if you're looking to spend time in a second destination, the Perk is useful -- but it requires a round-trip award.