I thought the tooltip of Synthesis stated that it would apply to any heal that Lifegiving Veil would normally affect, and Lifegiving Veil will certainly not work on other Chloro's actively using Lifegiving Veil.

The only way I found to make Chloros work, and able to heal each other was to have 1 hotkey for each Chloro that would remove their own Lifegiving Veil buff (and reapply it when pressed again, sort of a toggle) that you could press for the Chloro getting focused.

Lifegiving Veil has a 15sec Cooldown, however, so removing and reapplying it became too much to worry about.