I've seen this feature in a couple of beta firmwares but haven't been able to use it recently because those betas haven't been stable enough. It's in 3.8.2.3 though so I can use it again.
Draytek's session based load balance seems to work really nicely with steam downloads so I might be able to play Doom today
Here it is load balancing my slow ADSL with my phone's 4G hotspot: