He already has by agreeing to do them on the same day. God knows how that works out with the vicar, but even assuming he's okay with it, they've got to organise 2 sets of funeral cars, pall bearers etc. Unless they are several hours apart, say 10am and 3pm they won't have one set back home before the cars need to start for the next one. It isn't the sort of thing that can be rushed, making the bereaved get back in the cars to go home after the burial or cremation.
They aren't a huge funeral firm, they probably only have one hearse and a couple of cars. Hiring more will cost and they can't charge the extra on to the customer if they agreed to the date.
I don't think they'd have a muck up re coffins getting mixed up giving it is also kind of a tribute to John Bardon. Keeping true to Jim's character in having people remind us he was far from perfect is one thing, having a 'comedy' funeral could be seen as disrespectful.