Can I use more than one Chapter tactic if my army includes models from more than one chapter? I.e, if I use Dark Angels and Imperial Fists do the Dark Angels use their own and vice versa with Imperial Fists or are they forced to use just one or the other?
It depends on how you have them in detachments.
If you have them in all one big hodgepodge detachment, no-one gets any chapter tactics (they are too distracted by the other guys sexy abs and forget them).
If you keep them in separate detachments, then each will use their own traits.
Example 1: A brigade with mixed IF and DA
units, no tactics for anyone.
Example 2: A patrol with only IF and a Battalion with only DA
, each get to use their tactics.