I'm asking about whether we can gain a second love interest when we already have one in that playthrough. I'm not asking about going back and changing it, I'm asking whether we can have two love interests at the same time.
Nope. No multiple romance partner paths are planned, that'd be a mountain of extra writing and logic.
You can have multiple sex partners through brief flings, if you choose to keep Killigan single, but committing to a relationship (past the second confirmation point) will lock those off.
