I am guessing that what OP has observed has nothing to do with the number of FAs in the domestic F (J) cabin. What OP described sounded like the FA just was not competent enough to do domestic F service. If FAs are not competent to do the service, then one FA or two FAs would not have made the difference. Yes, one can argue that if one FA is not competent, then second FA can fill the hole and maintain the service.

I don't want to play devil's advocate, because I am under no circumstances AAplogist, but looking from the corporate side. LGA-DFW dinner flight, after drink service and meal service, the second half of the flight, pretty much most of the time FA would simply sit at a jump seat reading People magazine. It will not be an easy argument to assign a second FA to domestic F (J). I do think FA who is not performing to a set standard then needs to be pulled off from the duty and retrained. After repeated retraining does not improve the performance of the FA, then the job of FA is not for this particular person, needs to be let go. I understand that such is the standard at many non-U.S. airlines, but most U.S. airlines do not practice this.