I do it on farm all the time cause I'm bored. But once we start it on Heroic... single target all the way for me.
Only time I dot the non-kill head as SV is when I hit readiness and have black arrow off CD with the dot already up on the kill target (putting it on 2nd target increases proc chance of LnL, though it's arguably not worth the lost focus, seconds of no black arrow on primary target, etc). That's at most twice during that fight.
If he's using TotH for that fight (which I wouldn't recommend), I could see blue or green getting hit with a MS if he has a LnL proc with TotH procced and he's close to focus capping.
Demo Lock: 1 Global Cooldown to put DOOM on the second target, which if using Unerring Vision procs a fel Imp every 15 seconds; Fel Imps are the only reason Demo is decent at the moment.
Boomer: 2 Global cooldowns to put both your moon/sunfire up and get insane amounts of instant cast Starsurges which way outdamage anything that you can cast as a boomie.
Shadow Priest: is not the optimal spec, they don't use instant MS/MB anymore iirc, it's all about MF:Insanity now.
Hunter: should never touch another head except for the mentioned cleave during add-phase on heroic.
Also in the middle of a hectic fight(Lei Shen last phase?), it will be very hard to notice the proc.
---------- Post added 2013-04-17 at 04:33 AM ----------
You should not use 2 seperate black arrows on this as a hunter, instead what you need to do is use 1 black arrow, and 1 explosive trap (they stack) If the trap is placed directly on the serpent head, it will trigger, and do damage to the actual target.
There may be scenarios like having Renataki's Soul Charm where you want to use your Black Arrow on the last tick, but since I don't have that trinket yet, I can't really speak on how to use it optimally.
Tarren Mill|Kiea of Solidarity EU
Refreshing BA with Readiness gives you one free tick on the seconds BA (although you lose overwritten ticks from the first) and I can get my second BA hitting for well over 500k.
I tried MM and I found it not so good. The heads are at 80-100% for like 5-10 seconds, max 3 aimed shots..