I think the point the OP is making is if you have a joint mortgage, does it have to be paid from a joint account and what are the implications if it is paid from just one of the borroweres own account (so in effect, the other is not paying directly)
I don't know what the legal ramifications of this are and what would be the implications if there was divorce proceedings but I know from a practical perspective, I have a joint mortgage and joint CA but have also swept some excess funds from my own CA against the mortgage in the past via my bank's app quite easily. So from a practical perspective, it's feasible, from a legal perspective, I can't answer that.