I suppose that you are using the outside port of the ASA connected to a switch from where the two routers are connected to offer the to Internet connections.

In this case, you have only one VPN tunnel established. And it uses only one of the two connection. If you tried to load balance, the return packets is the job of the ISP.

To have to solution you are asking, you need 2 ASA which means 2 tunnels, and 1 router which balances traffic between the two tunnels.