/cast [target=targettarget] Seal of Light

[zenga]

this should work:

#showtooltip hotnakedchick
/focus target
/assist focus
/cast judgment of light

You target remains the same (tank you are healing), you set your focus to him, then you assist him and without your target changing you will cast judgement of light on the mob

afterwards your focus will stay on the tank, so you can make another macro with /clearfocus if you want to do that

edit= i think this might work as well, but not sure since i just logged out the game
#showtooltip elvispresley
/focus targettarget
/cast [@focus, harm, nodead] judgment of light