I've experienced and read about conflicts with S&D, while the Teatimer is active. Turning the Teatimer off often seems to resolve these.
I've had the occasional issue uninstalling or updating S&D with the immunity active. (Hint: De-immunize before uninstalling it.)
If the programs do not appear to conflict, and having uninstalled Spybot you notice no performance change, IMO there is no reason why you shouldn't leave it installed, or rather, re-install it.
You can have as many demand scanners (including A2 free) installed as you want. But as indicated above, only one of each type of resident. That's a general rule of thumb; a guide that serves most users reasonably well. There are always exceptions, of course.