Create a mangle rule to add a routing mark to the traffic sourced from 10.10.10.50
/ip firewall mangle add chain=prerouting src-address=10.10.10.50/32 action=mark-routing routing-mark=20.1gateway passthrough=no
Add default routes for both gateways, appending the routing mark to the 20.1 gateway.
/ip route add gateway=192.168.10.1
/ip route add gateway=192.168.20.1 routing-mark=20.1gateway pref-src=192.168.20.2
Note that you will need to exclude local networks from being marked. Add 'accept' mangle rules for each destination network with passthrough disabled. i.e.
/ip firewall mangle add chain=prerouting src-address=10.10.10.50/32 dst-address=192.168.10.0/24 action=accept passthrough=no