My sim was romancing a married sim (not in household) when he asked to propose to her so I said yes and the option to elope immediately was there so my sim married the non household sim and he is now married to my sim and his original wife at the same time. I tried this with and without mods and the same happened, also my married sim cannot propose to other sims. I'm pretty sure the sims 4 only allows one married spouse.
I've had this happen in some of my rotational played families.
Ralf Palm is now married to Rory Oaklow
and Christina Palm-Creep.
The game decided, that Rory (single) appeared at Ralf's house and wanted to take the next step in their relationship. Ralf accepted and they were able to marry.
