Focus is private to you. No one on the server can see who you have focused. The only thing you can do is have your alts use /cast [target=focus-target] DPSSpell when your main has the mob you want dead targetted. If your main isn't targeting the mob, what is it targetting? And if your main is focusing the mob it is attacking, why change focus like every 5 secs? My main has no focus and my alts focus my main. For healing I use mob aggro to heal:
/cast [target=focus-target-target] HealSpell
You can add in a heal anyone before that for emergencies:
/cast [target=focus-target, help] HealSpell

Hope that helps and good luck.