How does adding an AP to an AD work?

Not talking about sz, but when adding Rexulti to Wellbutrin, does it make the AD work better? Or is it the AP that’s working somehow?

I’m asking because I’ve been on Wellbutrin for about a month now. I’m on it for the side effect of decreased appetite, but I didn’t get that side effect until the day after I started Rexulti. Rexulti commonly causes weight gain and I cannot for the life of me find any mention of weight loss or decreased appetite.

So did the Rexulti cause the Wellbutrin to work differently?

Historically I’ve always gained weight super fast on every atypical I’ve ever tried.

100 lbs in 6 months on Saphris
40 lbs in 1.5 months on Latuda
10 lbs in 2 weeks on Abilify
45 lbs in 2-3 months on Risperidone

I’ve tried over a dozen, same for all of them. Expected the same of the Rexulti but am dumbfounded that my appetite is all but gone. I’m so confused by this! Happy, very happy, but confused. Lol