Far Cry 2

In the Southern Territory, you must complete a mission for the U.F.L.L. and A.P.R.. Right after you complete at least one mission for both sides, this achievement will be unlocked.

These two achievements (Playing both sides... & ...against the middle) are essentially story-based so they're hard to miss. In both territories, you could do missions for only one faction, the APR or the UFLL. Eventually, however, that faction will stop giving missions. This forces you to switch sides if you want diamonds, this achievement, or to finish the game.

The achievement unlocks right after you complete the mission for which you switched sides.

If you're going for the following achievement, you'll get this naturally:



Once you subvert a mission for the UFLL and APR, you'll unlock the achievement. All that's required to unlock it is that you work for each faction in at least one way, so subverting the opposite side's mission counts. If you subvert a UFLL mission, you'll have worked for the APR and if you subvert an APR mission, you'll have worked for the UFLL. So if you're working on your six Act 2 subversions, rest assured as this will come naturally.