And not a glitch exactly, but more an example of an EA development team not paying attention to their own product.if you have the Uni EP, a sim with Social Networking skill at Level 10 can use the Relationship Transmogrifier to force a sim to become their Partner even if they already have one.
What happens in Lucky Palms stays in Lucky Palms.
If you have the NRaas Traveler mod, your sim can have as many Partners (including spouses) as you have worlds installed, provided they like to travel a lot. So if your sim travels to the three WA worlds, goes to Uni, and Oasis Landing, they can actually have up to 6 partners as long as none of them are ever in the same world at the same time. The restriction of at most one Partner per sim is really on a per world basis. These include proms and picture taking in certain photo booths, and are widely regarded as glitches.
There are certain in-game situations that can provide a sim with a new Partner without resolving their relationship with an existing one. Okay, now that's out of the way, here are some 'atypical' circumstances. So a mod that finds a way to bypass such a restriction is likely to create more instability than fun. Steadies and higher (fiances, spouses) are considered Partners and, under 'typical' circumstances, a sim can only have one Partner at a time.There are many situations where the game's coding looks for a sim's Partner and expects to find at most one result there.