You need two things:
1) to pass through both the keypress & the shifted key press through to both toons. You could copy an entry from FTL DPS keymap in ISBoxer for this. Just don't include the FTL action in the the step - and possibly consider having the mapping go only to the druid. If that's still confusing let me know & I can paste appropriate screen shots.

2) a macro on the druid that selects the appropriate target. The easiest way to do the later in a two-box situation is
Code:
/use [mod:shift,@player2] [@player] <healing spell>
Alternately, you could explicitly target the dk
Code:
/use [mod:shift,@<dk name>] [@player] <healing spell>
The former has the advantage of working if you replace the DK; the later has the advantage of working no matter how many people are in the party.

More broadly though, you may want to investigate repeater regions and the use of Healbot, Clique or Vuhdu. That will let you heal additional toons without any additional set-up should you end up heal/tanking for a 5man.