The interface is just used for the special "failover-IP", which even the smallest server has - the bigger ones have 2 or 4 failover-IPs. I found some hints, but no clear answer, that the swich can also be done automatically.
The bigger servers not only have the special failover-IPs, but also an additional 16+ IP-Range assigned, which can be used for clustering.
ovh itself does not guarantee anything with clustering, because they had ready-to-use clusters themselves and want to bring that service back soon.
Only option I see so far is testing - I just ordered 2 isgenug. If you're interested in testing for yourself - we could share the month runtime, I just need the servers for one weekend to fully test all functions I need.