I started doing this with my friend. We are 2maning him each day.
This is how it goes:
1. Have 2 WoW clients open.
2. Make sure you and your friend are at the summoning thing.
3. On the second instance of WoW, login on your account, your first instance will be logged out. But the character will still be there. Just like when you AltF4
4. Get your alt inside, summon the boss. Then Alt+Tab to the other instance of WoW. Log back with your main(the char you want loot on, that is allready parked at the summonig stone).
5. Kill the boss. Receive loot :P
6. Repeat for other chars.
Also,this is for alts on the same account
This is somehow common sense, but I'd think I'd post it since some people might not know of this.
Also, the minimum required level is 65+ for your alts to summon the boss.
Edit: You need to be a maximum of 4 people in the party, not counting your alt, for this to work.
Cheers, and GL with getting the mount!