Let me rephrase your post to check whether I get the scenario properly. Both ether1 and ether2 are WANs (internet uplinks), and you want the SSTP server on the Mikrotik itself to communicate via ether2 while all the other traffic would flow through ether1.
If so, you indeed need an extra routing table that contains a default route via ether2, but since it doesn’t seem to be possible to make the SSTP server listen in any other VRF than main(at least as of 7.20.6), you have to use mangle rules or routing rules to make the SSTP transport traffic use that table.
The most generic solution is described in Static Default Route - I'm missing something - #9 by sindy (start reading from the last paragraph to get the context), but if you only need to make the SSTP server use ether2, a simpler solution exists:
/routing/table/add name=use-ether2 fib
/ip/route/add routing-table=use-ether2 gateway=gw.ip.of.ether2
/routing/rule/add src-address=ip.of.eth.er2 action=lookup-only-in-table table=use-ether2
That’s all - all TCP servers have no choice but to send their responses from the IP address at which they have received the corresponding requests, and the routing rule makes sure that whatever is sent from the address associated to ether2 will use the corresponding routing table.