I multiboxed 2 instances of HOTS on 2 different PC's. I used Input Director to broadcast the mouse and keyboard across both PC's. On Battlenet 1, I made a Tank and on Battlenet 2, I made a healer
You will need 2 Battlenets to multibox HOTS, as we are only allowed one HOTS account per Battlenet.
You will need Input Director to broadcast mouse and keyboard, as Isboxer does not support use with HOTS that I know of.
I was not able to successfully get Isboxer to launch HOTS, which is why I used 2 PC'S and Input Director
For the Primal Flamesaber mount just team your 2 characters together, and run A.I. Battlegrounds. In A.I. Battlegrounds its just your 2 characters and the rest of the team and the opposing team will all be computer A.I. characters.
Use broadcast on, to move your characters around by clicking either the minimap (perfered method) in the lower right corner of your screen or click the terrain directly. You can then set Hotkeys for the 2 characters toolbars where you need them to perform a decent broadcast of rotations between your Tank and Healer. Abilities in HOTS are not customizable as to what toolbar button a certain ability can occupy, but you can change the hotkey for those buttons. So button 1 on PC1's toolbar can be 1 and on PC2's you could make it 2, or 3 or whatever. Also in game, in the "options" set "Mouse and Keyboard" options for "Confine Mouse Cursor" to "off" make sure "Enable Camera Follow" is check marked and "Enable Smart camera Pan" is unchecked.
There is a way to launch more than one instance of HOTS on the same PC using a program called Sandboxie. But Input Direrctor can not broadcast to different monitors on the same PC. So you will have to move your mouse back and forth from each game window to play both on the same PC.
Be carefull to not let your characters remain motionless for to long, or they will leave the battleground. Characters that leave the Battleground will get no credit. If one of your characters gets kicked this way, and the other character completes the battleground, he will still receive credit even though both are no longer in there together.
Hope this helps,
Ridosaw