Add 2nd map to 1/1

[Miritellane]

Hello
I want to "add" a second part 1/1 to my 1/1 scale card, but externally, as a dlc, or .scs, or ??
In fact, a friend starts mapping, and we want to be able to add his map to mine, as we do on ETS2 with the mods.
We tried with my map in profile, then his map in mod... and it crashes... the internal links and the .sii are correct, but we can't figure out how to handle this montage...
Does anyone have an idea or a solution?
Thanks in advance