yes it is, and no there shouldn't be any problems, at least with windows XP
I normally have a wired keyboard plugged into my PC, in addition to a wireless keyboard and mouse set (Logitech desktop MX), and recently due to problems with the mouse had an intellimouse explorer also plugged in (and for a few minutes had an MX3000 set also plugged in
).
The only problems you might get are if you knock the mouse, or leave something on the unused keyboard (happens to me quite regulary), or if you install software for two different keyboard/mice you might get a conflict.