You should allow the party to split, if that is their consideration. "Do we focus all our strength on this one concern or spread it across multiple concerns?" is a conceptually valid choice for them to make. A siege is not a dungeon. "Don't split the party" is tactical advice for navigating a hostile, unknown territory, where player characters have no other backing and are liable go get lost or picked off one-by-one if separated. By contrast, being a defender in a siege takes place in what's ostensibly home territory, when a party splits, they're spreading to known locations, with backing from friendlies.

For example, if enemies have a choice to attack from multiple directions, the question naturally arises where to place the defending troops. Granted, not all of these are true dilemmas - meaning there are better and worse options, the challenge is in correctly assessing them. But that's normal for any game with non-trivial strategy.