Medium yellow, as in medium-sized yellow, or medium yellow as in medium yellow color? Not that it matters... The yellow and the sailfin are both Zebrasoma genus, as is the purple (flavescens, veliferum, and xanthurum respectively).
In theory, tangs of similar body shape or color can get, umm, feisty with one another. Therefore, in theory, your two Zebrasomas (yellow & sailfin) could potentially quarrel.
I asked a similar question regarding my 120g tank, and was advised to stick to just one tang (wound up with a Z. desjardinii, though some ichthyologists would surely say that he's a veliferum...).
How large is your tang tank?