I believe allies should be allowed to share tiles. By this I mean if I need to go south on my allies road, I should not have to get off the road, and then back on, just becuase he has a worker irrigating it or something. If I am allied with some one we should be able to have both of our people on the same space. This would really help with coordinated attacks. Additionally it would allow you to stop your people off in their city to rest up in the barracks or protect it from enemies while they build more troops.