One good trick to make communion slaves Last longer is to give them reinvigoration somehow. If they are sacred, an earth 4+ blessing might prove useful. Otherwise, boots of the messenger (+4 reinvoration) never go out of fashion, relief is a great relief (and a master with 4+ slaves only needs 1 nature to cast relief, not that you should ever limit yourself to 4 slaves when 8 will do so much better in sharing the fatigue
), and since spells affecting the master affect the slaves, you might even get away with having a master cast earthpower.