Would it be possible to topcrop this new batch and pitch a bunch of it into the stuck APA? Would this possibly bring the gravity down?
Technically yes you can top crop from your new batch, however if that new batch doesn't ferment below 1.020 either then you don't want that yeast to help further attenuate your first beer because it simply isn't going to. If you harvest yeast that becomes accustomed to chronic under-aeration then that strain will gradually mutate into a strain that under-attenuates future batches and takes longer to ferment due to low viability.
What sort of wort aeration practices are you using? Under-aeration and under attenuation typically go hand in hand so I think the problem may lie there.