You can do some outgoing policy in few ways.
General MultiWan I write here http://forum.mikrotik.com/t/moving-from-fail-over-to-using-both-wans/143145/2
But this will be hard to do a proper way “Balance” traffic.
I use often LTE stuff and one way is do a cyclic speedtest and for avg from last 5 I do some QoS and LoadBalance policy but this must be with some scripting and it’s not a out-of-box solution.