you can do a /assist <nameofyourDKtank> at the beginning of your DPS macro and they will assist him. The advantage of a FTL system is that you can swap mains quickly (just by changing to a new window) and you can use your focus for assigning a "burst" target or CC target, neither is necessary for heroics. If you want to arena a lot then FTL is very good, since you can assign focus to their healer, have everyone target someone else, and then quickly swap DPS targets without the other team "knowing" until its too late.

If all you want to run are heroics, then there is no reason to do FTL (imo), just makes everything more complex and there are no situations in which you are going to have your tank die and your team keep going (in clutch times when boss is almost dead and DK dies, you can still just keep boss targeted with your dead DK and spam attack keys and hope boss dies before team does). Stick with your tank having his tanking macros, then all your DPS macros would be something like:

/assist focus OR /assist nameofDK
/castrandom etc, etc,etc

You can set focus to your tank if you ever want to have multiple tanks swap in and out, or you can just hardcode the name - doesn't really make much of a difference imo. Either way will work when you are solo too, although you will get minor UI errors that focus is empty or nameofDK is not in party.

Remember Jamba has stuff to dealing with name assignments in its macros and auto master switch when focuses are set, its powerful stuff and can be used with this. I have a prot pally and a DK tank and I swap betweem them all the time, all my DPS macros are exactly the same and I just have a single macro that assigns focus and Jamba takes care of the rest.