Is there any reason not to put two access-point, each on a different channel close(within 15ft or so) to each other?
To clarify this, two access-points on different channels placed 15 feets apart will not cause any interference with one another?
You are correct.
Fifteen feet should be more than adequate as long as they don't have directional antenna pointed at the other, and that each is adequately shielded.
Don't forget the minimum Channel spacing, is not always enough. So, for example, if you had channels 1, 6, and 11 to choose from, it would still be best to use channels, one and eleven to truly minimize interference.
Any Enterprise quality AP will be sufficiently shielded.