Really necessary? No. But there's a huge "but". Since you're basically connecting the two scanners together directly (using a "T" coupling, most likely) it will be without any isolation (the multicoupler includes isolation) you could easily have some signal mixing or other issues that will cause signals where they don't actually exist or other artifacts that may cause issues. You'll still have loss since the signal will need to be split between the two devices. The multicoupler splits the signal prior and then isolates the two sides from each other and prevents the internal signals from one scanner being fed to the other scanner. The "active" splitters mentioned add a preamp to the mix to eliminate the loss. This preamp may help or it may hurt (close or strong signals may overload the preamp for example) so it's not always a good thing.Is a Multicoupler really necessary to run two scanners simultaneously on the same antenna?