I've got three scanners in my garage I listen when working there. One is a portable and monitors a local system, thus not needing any increased reach from an external antenna.
The other two are mobile scanners, but on a single 12 V power supply. I looked into getting the multicouple mentioned above. When crunching the numbers, I found it to be less costly for each scanner to have its own antenna in the attic.
At the fire station, where we had access to a taller tower to which a scanner antenna could be mounted, we did use a multicoupler. Several scanners are "supplied" that way. Given the length of the antenna cable run involved, we used LMR 400 cable from the antenna to the multicoupler.