I've come accross this before... the same mixer/engineer/something did both bands on the split, but I can't make it appear so. when I first add him/her I have to choose "sides", and if I try to re-add him for the other side, I get a message "This artist is already present in this line-up. To add information, just edit the line or click the + icon ("add year range / instrument") on the lineup edition page." but unless I'm missing something there's no option to add/change "sides".

if it helps you visualize the situ, I'm referring to the candlemass/entombed split as an example.

Yeah, this split has the same problem. The site owners are aware of it.

shall I write something in the notes, or ... ?

I suppose until the issue gets resolved, yeah, that would suffice.

What Obscurum said.