Weight is not a radio problem, and will not factor in whether a system is operating properly or not. You need to consider the weight range specified for the model by its manufacturer. If you're within their max weight recommendation, you'll be OK. Even a little over won't hurt anything. Many models fly perfectly well even when a little overweight.
When using two battery packs, you really need to make sure they are isolated from each other so that the failure of one cannot cause a problem with the second one. If you just have two packs connected to the receiver without any kind of isolation, then the failure of one pack could cause a failure of the system. In that case, just use a single bigger battery pack to have the duration you'd have with two packs. If you use a switching device that isolates the two packs, then you'll be OK.