Hello! I have a dual wan configuration.
Example:
ether1=ISP1
ether2=ISP2
ether3=LAN

In Interface -> pptp-client, I have created 2 PPTP Client interfaces (PPTP-ClientVPN1 and PPTP-ClientVPN2), which point to the same VPN PPTP Server.
By default, all traffic from the router to the Internet (Router <-> Internet) goes through ISP1 interface. Traffic from my LAN (LAN <-> Internet) is load balanced between ISP1 and ISP2. Because PPTP Client connections are initiated from the router itself, their traffic always goes to ISP1 interface.

I want that PPTP-ClientVPN1 traffic always goes to ISP1 interface, and that PPTP-ClientVPN2 traffic always goes to ISP2 interface. How to do this?

You must define other pptp end (if is not the same) on what gateway are reachable, by routing rules.

If are the same, you can not do that, or you can if you add another Router....