Well for starters I would make sure you have "Talwin NX" tablets/pills as there is a very similar sounding preparation called "Talwin PX" which you would have no problem with assuming you have the proper implements for filtering and injecting properly. As we always say on bluelight; injecting pills is dangerous if you use anything besides a mircon filter. But if you must, I think you should look around to see if you have any of the literature that is handed out by the pharmacy when you pick up such a medication. You want to look for the inactive ingredients to see what all is in there.
I'm not so certain it would be as simple as a cold water extraction or such to separate, but you should see which are soluble in what (Water, ethanol, acetone, formaldehyde, ether, etc) and if there are any between the two chemicals (Pentazocine and Nalaxone) it is possible, however; I am not an expert at such separations between two drug molecules and just recommend you double and then triple check what you have, and see if you can't get an ingredients list so someone else (or you) can see what would be suspended in a solution, if indeed that is the right way to go about this.
Edit: Just re-read and I see nothing about how you are preparing this for injection; If you are not, there is a tablet out there called suboxone that is well known to contain both an opioid agonist and antagonist and taking it in sub-lingually (below the tongue) can defeat the nalaxone in it. (So can IV'ing it but I'm not going to encourage IV of pure antagonist with a pure agonist vs a mixed action like suboxone.)