As far as I know, it is not yet possible. AVM had once hinted that it might be added with a future firmware.

It can at least switch automatically to the other internet connection in the event of a failure (in my case from mobile to cable) or if the speed becomes very slow (but I could not simulate this). But it did not go completely unnoticed. The connection was gone for a while (minutes). The SIP phone could not switch cleanly either. But anyway, I wouldn't call it load balancing: it's simply a fallback.