Hi friends
I have two different wan that provide internet for me. each wan has 40Mbit bandwidth
I used mangle rule. and send traffic to both of this WANS and now I have 80Mbit bandwidth totally
but really we don't need this bandwidth and also WAN2 cost is more than WAN1
So I want set a priority on may wan for example if traffic usage in wan1 is low , its not necessary to use WAN2 at all
but if WAN1 become full , then I should use WAN2 and send some traffics to it

could you please tell me how I can do it ?

I should say , WAN1 bandwidth is not 40M fix and maybe in some hours it have 15M bandwidth , so I cant use traffic monitor trig toll for implement it .

There a couple of ways do to this, using PCC mangling just assign more sessions to wan1 over wan2 or use load balancing mangling and assign BW as desired.
Dont think there is a way to do it my actual load being realized but others may know......

pcc mangling - https://mum.mikrotik.com/presentations/US12/steve.pdf
load balance mangling - https://mum.mikrotik.com/presentations/US12/tomas.pdf

The latter method comes closest to what you want, read the presentation and decide.