No your correct. I guess the biggest issue would be doing the whole autorise between 2 parties as if the DD came out of your account and it was £60 and you pay £30 and Y person doesnt have £30 it could bounce.
Also that person would need to authorise your DD to take half automatically without control over the DD amount as it would be in your name. It would be fine for fixed bills but say power etc it could get messy.
Although thinking now a DD usually alerts you before the amount is drawn from your account a day or 2 before, a window then to say to X person dole me over half would be nice or alert them to approve their half the balance.
A quick way would be to at least mark a certain debit as a split with X person and when the amount gets notified to be taken you could hit a button to send the request for half and the other person could hit the pay before the amount is taken.