Each graph is going to work differently (makes/models/transmitted freq). Basically, interference between graphs may occur in two forms. One will be interference or clutter on the display while the other will be in masking echo's that you may want to see. Either case, you’re receiving returns that you don't want. However, you might not be aware of the interference when the visual clutter is not present.
If you're going to use two graphs, the best way to work around this would be to use two different freq graphs. Since they're sending different freq's and receiving different freq's, you won't have the interference problems. Lowrance makes some good deep water graphs that operate at lower fewq's ("DF" model's). You could use the 50kHz on one graph and the 200kHz on the other (you're pref) Or you could just get a graph that operated strictly at 50kHz. I think most of the DF/50kHz models transmit more power because the 50kHz is more suited for deeper, ocean uses. Because of this, they typically transmit more power than you might need for fishing our lakes. Using too much power with manual sensitivity and ASP modes can result in second time returns. Have you ever seen two identical displays displayed on your graph (one above the other)? This is from second time returns, when signals bounce off of the bottom, come back up and then bounce off of the bottom of your boat or the surface clutter and then head back down to produce the second display. This is from being way too sensitive or using too much power (but we can't choose power settings on our graphs so we need to decrease the sensitivity)
If you're using two graphs on the same freq and don't notice any interference on your displays, you might want to check for receive processing interference (jamming) issues. You can do this by taking a spoon or anything you can mark in deeper waters and use ONLY ONE graph (the second one should be OFF). Mark your spoon at a depth and then adjust the sensitivity until you the spoon is no longer marked. Then bring back some sensitivity until you just start to mark your spoon (use just enough sensitivity to mark your spoon at a depth of say 40'). Now you can turn on your second graph and see if you can still mark your spoon on the first graph. The sensitivity of the second graph doesn't matter because we're looking to see if it causes interference that hides your spoon on the other graph. You can try this for the other graph as well. This important in cases like d a shared with us the other week. That "thicker" mark on the bottom of the lake might not show up if a second graph is used that causes masking of returns.
I’d suggest using two different freq graphs unless you’re fishing in conditions that peak sensitivity wasn’t an issue.
Just my 2.5 cents…
Ray