I’d like to run a SSTP session within a PPTP session.
Had a look at /ip route, doesn’t look like there is a way doing it there. Had a look at mangle, perhaps that’s where I can redirect the packets, but prerouting chain cannot use output interface as a condition. So I am out of ideas…
Select it for what? You just make traffic to the SSTP end point route over the PPTP interface and then make all the other traffic route over the SSTP interface.
This would be the easiest way to do it without any mangle rules, if you aren’t worried about other traffic to these IPs not going through the VPNs (assuming PPTP and SSTP are different IPs)