Board Thread:General Discussion/@comment-24825179-20151012055323/@comment-25926288-20151012123230

Nope. The Wand was the light and Zelena was the Dark.

So, it works in two steps:

1) The Wand needs only dark magic to cross realms.

2) It need strong dark magic, specifically.

Regina's dark magic has apparently waned and is not totally dark. Zelena uses solely dark magic and hers is strong enough to do the spell.