Yes, certainly I tried this (see initial post), but RouterOS seems to be confused by the same gateway IP address.
I guess I found Chupakas Tutorial on Routing Failover you mentioned, but I doubt that this will work with the same gateway as the same problem persists.
I finally found a configuration that comes close to what I wanted to have. I described it in this new thread where I directly requested a change to prevent the route confusion in RouterOS.
Summary:
- add-default-route=no on both interfaces
- two static routes to the gateway, 192.168.144.254%wlan1 and 192.168.144.254%ether1
Best,
Daniel