There are plenty of sites where multiple modulators are required, but when you're installing them, there are specific settings you have to change.
These have to be unique for each modulator.
Output channel - i.e. UHF 39
LCN (Logical Channel Number) - i.e. 101
Channel name - i.e. CHANNEL-1
Program number - i.e. 101
TSID (Transport Stream ID) number - i.e. 1000
As for why? If you don't care why, then read no further.
Simple enough, you can't stack two modulators in the same 7/8MHz channel space. Not without using modulators that do this internally (like the HD2000-series and so on), of course. Use your digital signal strength meter, choose free channels, and you're on the way.
This is the number you see on the TV screen, i.e. 24 = ABC News 24, etc. The default LCN for most Zycast/Resi-Linx modulators is 101. If you have two modulators, both trying to use LCN 101, then what might happen is confusion ("How do we have two 101 channels?"), TVs not tuning in, or the TV may randomly assign another LCN (350, 351, etc.).
Again, using the same name for two channels (i.e. CHANNEL-1) isn't necessarily a thing that will kill the system, but the confusion alone is worth avoiding.
*Most* TVs couldn't care less if multiple modulators have the same program number. It's not anything the consumer sees, so leaving it the same is *normally* okay. However, some rare TVs get confused, so why not just match the LCN to the Program Number, and avoid the issue?
OK, this is the other really important one, and is the one that most people miss. The short way to think about TSID is that every digital channel has one, and unless they're unique, most TV tuners can get very confused. Use 1000, 2000, 3540, whatever you like, but make each modulator channel unique.