For my short haul flight the online check in service says that I can add the second bag for 70 EUR. Not sure if I can actually do it (as it is an OTA booking and the first thing the page says is that it cannot be modified online, only via the call-center). It also tells me that the second bag will be my last one. More is not allowed.

Now I will first try to pack in a single bag before I experiment with this further

UPDATE: I did pack into a single bag in the end, so I will not learn how this works. No report from me.