Thee would be an element beyodnd your control because the other person would have to be asleep for a shared dream, but I don't think there would be any way to test whether they are asleep. SO there is no way to tell the differetce between a time when it is impossible and an actual failure. If you allow for the possibility that they might not come, they might not even if asleep... otherwise, you'll have no option but to get a dream character if they are awake.

Also, I don't think you would be able to get a shared dream unless you made your dream the same as the other person's - you can' share a dream when you're dreaming about two different things. Perhaps you should give the other person control of the dream? It makes sense to me that spinning and deciding to go to the other person might work, but have unpredictable results if the other person is awake. Unpredictable is still harmless though, except for some frustration.