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...

Anyone could give me a direction how to go about?

Yes it's possible - Just route your SSTP tunnel down the PPTP interface.
Not sure I'd recommend it though!

Yes it's possible - Just route your SSTP tunnel down the PPTP interface.
Not sure I'd recommend it though!

Thanks for getting back to me~

in /IP route, gateway I select PPTP, but then how/where do I select SSTP tunnel?

or should this be done at other place?

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.

/ip route add dst-address=<pptp server IP> gateway=<your WAN interface or gateway IP>
/ip route add dst-address=<sstp server IP> gateway=<PPTP interface or remote IP>
/ip route add gateway=<SSTP interface or remote IP>

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)

MTU settings may also need tweaking

/ip route add dst-address=<pptp server IP> gateway=<your WAN interface or gateway IP>
/ip route add dst-address=<sstp server IP> gateway=<PPTP interface or remote IP>
/ip route add gateway=<SSTP interface or remote IP>

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)

MTU settings may also need tweaking

without these CLI i think i could never figured it out by myself. really thank a lot~~

the pings are very unstable, could it because of MTU? if so, any suggestion of what i should try or read?