Board Thread:General Discussion/@comment-26127761-20160331033430/@comment-27701762-20160331034530

The premise would be that the requirement for passing through is not "a human soul and a monster soul must pass through at the same time," as though unlocking a door with two keys. Rather, the requirement is that the human and monster souls must be merged.

There's also the issue that Asgore needs Frisk's soul, so even if the barrier worked that way, Asgore would not agree to it.