Two proxies simultaneously to increase bandwidth

[Ulysses_]

I have two linux VM's, each running a HTTP/HTTPS proxy. The two proxies send data to different networks over the internet, through proprietary tunnels.

I can make the proxies transparent and use them simultaneously from a pfsense VM with multiwan to increase bandwidth. However, HTTPS does not work this way and you only see HTTP sites.

What can be done to take advantage of both HTTPS proxies simultaneously for more bandwidth?