From the Asus Dual WAN FAQ, dual wan load balance mode will not increase SpeedTest beyond 1Gbps for any single connection session. Take note of the last sentence.
https://www.asus.com/sg/support/FAQ/1011719/
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Load Balance Mode:
While enabling the Load Balance Mode, you can distribute the load between the Primary WAN and Secondary WAN. In this mode, you can also configure a particular device to use either the Primary or Secondary WAN.
For example, you can configure your game console connected to your local network to use the primary WAN and the smart phone to use the secondary WAN. Additionally, you get more options to route requests through either the Primary or Secondary WAN.
By enabling routing rules and adding rules appropriately, you can route requests from a particular device through the Primary WAN or Secondary WAN.
Note: The system will separate Internet sessions based on the load between the Primary WAN and Secondary WAN, instead of the overall bandwidth or overall data rate.