@phil.davis:
Each single state ("thread" of download/upload) has to live on a single WAN. To the upstream server the packets back and forth have to come from/to the same WAN IP. So a single-threaded download can only run at the speed of the WAN which it gets allocated to.
When the WANs are equal bandwidth, the strategy is to put each new connection onto the next WAN, spreading then umber of connections on each WAN equally. Statistically then the WANs should experience about equal load (e.g. if there are 50 "connections" on each then it is likely that the "connections" that are actually wanting lots of bandwidth will be spread between the WANs).
If there are only a few connections, then maybe the ones that want bandwidth happen to have been mostly allocated to WAN1 and those that are more idle are on WAN2. In that case, bad luck, WAN1 will be saturated while WAN2 has free bandwidth.
If there are significantly different bandwidths on each WAN then you need to put weights on the WANs in the gateway group (they are on the GUI), so that most connections get allocated to the WAN with more bandwidth.
If you use a download manager that downloads bits of the file in parallel, then it will make multiple connections and those will (most likely) be spread around the WANs in the gateway group. So you could see total file download speed near the summ of the WAN speeds.
Note: You did not mention doing anything with rules. You need to put a rule on LAN that will feed traffic destined for "the internet" into the gateway group. Without doing that, all your traffic will just go out the default gateway.
@phil.davis thanks for explanation,now it's much more clear for me,you are right i forgot to mention about firewall setting,i did configure as you mentioned, with IDM downloads speed makes different (it uses two WAN at the same time).
Thanks for your time.