I've shared a pudding with a friend in Sticky Fingers more than 10 years ago. We were both totally stuffed from the previous courses but wanted to try their famous bread and butter pudding...............
I had a seafood sharing platter few weeks back. It came with two plates.
Sharing a pudding is something I've always done. I don't really have a sweet tooth so it's pointless me ordering something I'll only have a couple of spoons of.
I don't even mind people nicking chips off my plate :-)
Sharing a dish if pre-agreed is ok. Being asked to share by someone who's changed their mind because they've seen my order and 'fancy a bit' is out of order and a fork in the forehead is not an unreasonable reply imo...
My 3 daughters and myself often order a different pudding each, and then take some, and pass the bowl to the next person, so we each have some of all the different types! Greedy - us - never!