I don't think there is any real benefit for a 2 year old to go on a sleepover. They have such short attention spans and low tolerance, I can't imagine they would want to actually play together that whole time and enjoy themselves. Plus my 2 year old is still up a couple of times a night. So if it's just for a play date, no, I wouldn't allow it.
However, I allow sleep overs at grandparents house sometimes. Mum watches him when I work and if I have to start at 6am it's easier for everyone if he just stays overnight, plus occasional date night sleepovers. If I didn't have grandparents to watch him and a trusted friend offered to take him under the same circumstances, I would allow it. But certainly not just for a play date.