Simply put: it's probably better to use your low level Fir as material for your high level Fir.
The only exceptions I can think of:

1) Your higher level Fir is about to reach maximum level soon, meaning you will evolve her soon. In this case, it's slightly more efficient to combine the two immediately after you evolved your high level Fir, since the experience gain from using girl cards is so little, that it's only noticable at low levels. (Evolved Fir would be back at lvl 1, to start leveling again, but with way better stats and an extra passive ability)

2) You don't have enough gold and silver girls yet to fill all your squads. In this case, it may be worhwhile to keep both for a while, but keep one of the two below lvl 20. Eventually, you'll want to merge her into the higher level Fir, because you simply found better girls to put on your teams. Leveling them to 20 is cheap, beyond that is noticably more expensive, and because of that, a waste of resources for this kind of place-holder dupe girls.