I second that, GoldenAge. These people are all locked in the past because, presumably, they have no incentive to sort out their lives properly, they've just slid sideways slightly and both parents have long since been co-habiting with other partners.
What is well, weird, is if the DS was maybe 10 and he normally received presents from Mummy and Daddy, there possibly wouldn't be any pressing reason to change that arrangement - for him. However, the birthday boy in question is middle-aged, has his own children (?) and is well aware that his parents live separate lives. Why on earth can't they send cards, presents or money separately? Who are they trying to fool? Does clinging to the remnants of being co-parents make them feel better? I feel it's a bit cringey, and if I were the OP I wouldn't want to have any part of this idiotic arrangement, I wouldn't want to hear about it either, since it didn't involve me, and I would have said so.