Semi with separate gutters, neighbour asking to join gutters

[harryagain]

The normal way here is one has the front pipe and one has the rear.

[Dave Liquorice]

(assuming

This is the way to go, water coming down the roof does not hit the top of the end stops or down the small (1/2 inch) gap between them.

As others have said don't share "services" with neighbours unless you really, really, have no option. Even then such shared things ought to be in the deeds and the procedure etc for dealing with and paying for problems laid down.

[John Rumm]

I would opt for a shared system since it gives extra downpipes in case one gets blocked.