Living together and sharing bank account

My ex and I have been living apart for almost a year but have no written separation. He began publicly dating not long after I moved out and moved in with his girlfriend a few months ago. They have also opened a joint bank account. Is this allowed? I also believe she influenced our decision to separate and I know her phone number is on our bills before I moved out. Any action I can take against him?

There is nothing illegal about his sharing a bank account, as unsettling as it may be.