As the people on this forum have found, Path of Exile is a relatively difficult game to multibox. For one, there is no auto-follow/assist/aim. Lack of movement keys removes the possibility of formations. The characters don't stack, but rather bump into each other like psycho billiard balls. Another issue is the different movement speed of each character, forcing you into using 5 of the same class (unless you figure out a perfect balance between the classes' movement speed or something). There is a big problem with the characters spreading shortly after moving. Spread too far, and it can be a certain death sentence.
To get my toons together after they spread, I would run into a convenient wall/corner. When no wall or corner was available, I would take control of each toon separately (using a second set of keyboard/mouse), and move them close together before advancing. However, there is a simpler way to control each character individually from your main screen!
This is how I set it up:
Step 1: Key Maps
Step 2: Always On
Step 3: Right-click "Activate Repeat" and press "Make Copy" - rename it
Step 4: Click on the newly made copy of repeat.
Step 5: Under Steps, click on the first one
Step 6: In the lower-right box, under "New Repeater Target", choose the character you would like to control from your main screen.
Step 7: Check "Block buttons from originating window"
Repeat Steps 3-7 for each character you want to control.
As you can see, I setup 4 different repeaters with their own hotkeys. When I press F1, the repeater activates on a specific character slot while being blocked from my main screen. This allows me to move only the character bound to F1 by clicking around the character on my main screen. I can now move each of my toons individually from my main screen. "New Repeater Target" allows me to choose character slots from my 2nd pc, so using multiple pcs is not an issue here.
All in all, this is not a perfect solution but in my opinion it is a step forward. Another option I found was using VideoFX to generate live feeds of each window next to my main screen, and then controlling each toon from the video feed using a repeater region. Alas, VideoFX does not work with multiple PCs, so this does not help my particular case. How else do you guys keep your toons synced efficiently?