[mwan3] Is it possible to use second wan only when first one is saturated?

I have two wan connections. The first one is 100/100mbit, good connection quality. The second one is 50/8mbit and has a lot of packet loss.

I would like to route all the traffic through first wan, until it's badwidth is maxed out, then use wanb to balance the load. This would give me a good quality connection, but take advantage of the added bandwidth under heavy use.

Is there any way of achieving this?