I fear that's not possible. As RouterOS always want the route (and that includes the default route) to be configured by entering the next hops' ip address, this won't work (as the gateway is the same on both uplinks).
This WOULD work, if you could configure an interface based default route like Linux can do with something like "route add default dev ippp0", i.e. tell the router to send the packets out over a specific INTERFACE, rather than to a specific next hop.
This would be really helpful (and is of course only sensible for point-to-point links). But my feature request for that was answered with "we won't make this anytime soon" some weeks ago...
Best regards,
Christian Meis