toshiesumiI think it's depending on how you test the bandwidth. My assumption is if the test source is one machine (one IP) and the test server is located at one IP address, the test traffic would likely go through only one side.
simonorchI think perhaps you might want to look at IPSec aggregation https://docs.fortinet.com/document/fortigate/6.4.3/administration-guide/779201/aggregate-and-redundant-vpn
Fullmoonwell its a typical router/nat device wherein if you have 2-3 wan links its aggregates the bandwidth. pretty straight forward if you do speedtest on your computer you really get the desired speed of 3 ISPs.