The two rules are:
1. If you spend luxon faction, any kurzick faction you have gets erased in equal amounts (and vice versa of course)
2. If you have more faction with one side, the merchants and faction traders on the other side won't talk to you.
Which translates to this best strategy for game completion:
Do all the missions you can with one side, sell all your faction for shards before doing missions on the other side.
|