Fish finder conflicts

Hey everyone!

Thinking about getting another fish finder for the front of my boat. Either
I read or heard something about checking to make sure the two wont' conflict
when running at the same time. I've looked on Garmin's site, and a couple
others but didn't find anything about what to look for when buying a second
sonar unit.
So I guess my question is, what do you look for? What works and what
doesn't? Do you just have to look at the emitting frequency's and if so,
what are they in the spec's they give?

Thanks for any insight into this as I have a trip coming up, and need or
want to get one before...

TIA,

Doug