There are no immediate plans for this, sadly, since the complexity of handling sideblogs is increased over how New XKit works. The good news is that it should handle multiple accounts flawlessly; you can use different Tumblr accounts in different container tabs with no issues at all.

It's not ruled out though! It's just difficult to implement in a way that isn't horrifically clunky. I might have to build a whole new internal module for it.