Can you use them both at the same time? Yes. Will it improve anything? With those two cards not even if you tried real hard. For starters the HD 4890 is so far ahead of the 8500Gt in performance that it isn't funny. That and even if it was possible to combine the two it would work to the lowest common denominator, meaning the 8500 would drag the 4890 way down.
What you are thinking about is SLI or Crossfire; which only works when the cards are of the same make, model and specs (and as osholt said supported by the motherboard). About the best you can achieve by using both of those cards together is to run more than two monitors at a time if you were so inclined.