Board Thread:General Discussion/@comment-27565137-20160604195422/@comment-27392440-20160605010739

Simple answer here, using bankai puts immense strain on a shinigami and if the vizards are anything like Ichigo in his second fight with Grimmjow they have an immense strain on their powers in using hollowfication (Ichigo seemed completely out of energy at the end of his mask's time limit) so it makes sense that using both at once may only give a few seconds of power before making a vizard collapse, making it highly suicidal in actual/practical combat.