truuuuuuu (⊙…⊙ )
Ah, not necessarily. Different Omegaverse stories have different rules. There is no one set of rules that defines them all. In some of them, the alpha can have a bond with an omega and still have sex or even multiple bonds with other people (omegas, betas and other alphas alike). And it would not be far fetched (as some of them do have omegas that do top), to assume that even with a bond, while an omega cannot have someone top them because it will make them sick (and again, in some of the mangas, that only happens if the omega is in heat at the time of sex), that doesn't necessarily mean that the omegas can't top someone else. It all depends on the individual author's choice and the rules geared toward that specific story.
Basically put, YES, the brothers can be a mated bonded pair and still have sex with the Towa, if that is how the author wants to present it in this story. Heck, I even read one where two alphas bit the same omega at the same time, so that they could have a "triple" bond.
The twins are bonded together...
Seiji the ALPHA can have sex with other people and can have multiple mates
But Aiji the OMEGA cannot have sex as the bottom other than with Seiji (I presume he can only TOP others but CANNOT BOTTOM to others cause it will produce unwanted symptoms)
Aiji can have sex with Towa cause he is the TOP and he only BOTTOM to Seiji...
Do you mean Immoral Sandwich? Do you know how the manga ends?
I think Aiji and Seiji are not mates like Towa thought, because if they're mates then they wouldn't be able to have sex with Towa like they have been doing, and Seiji wouldn't react to Towa's heat either