Can you use multiple terrains in the same scene?
Yes!
Yes, but it is usually not recommended as it is resource intensive to do so. I have not tried myself, but that seems to be the consensus.
I disagree. Two terrains with a resolution of 500, will be just as light as one 1000 resolution terrain. And with it split into chunks you have more control over things.
Well, maybe, but I am trying to represent 12000x12000 meters, so I’m starting to think I just need to adjust scaling of everything. Problem is, I can only import, at best, a 2094x2094 heightmap, which means I lose about 25% resolution to scaling.
Nobody ever said that…there’s specifically a Terrain.SetNeighbors function which is only useful with multiple terrains, and I wouldn’t have sold as many copies of my terrain stitching utility as I have if people weren’t commonly using multiple terrains. 
–Eric
Thanks, Eric, for the input. I was actually planning on using that tool if multiple terrains were practical enough.
Oh I must be mistaken, I remembered that when people said about using multiple terrains it impacts performance. I guess that might have been for levels with sand/snow to have the depth of walking on an invisible terrain below the regular one for collisions.
Well, naturally multiple terrains will use more resources than just one, but not enough that you wouldn’t do it. Primarily you’d be concerned with memory, since LOD means distant terrains wouldn’t use much CPU/GPU time compared to the terrain you’re standing in the middle of.
–Eric
Glad you mentioned that, Eric. Because I couldn’t get the stitching utility to work properly, I abandoned multiple terrains and went with on large terrain. Granted, it is only large in the sense that I set it to 12000 x 12000. The actual HM is only 1024x1024. The size of the terrain doesn’t change the memory it consumes, does it?
No, only the resolution affects the memory. In what way was the stitching utility not working?
–Eric
Still has a seam from certain angles and creates a noticeable ridge where they stitch together. I tried messing with the…I think it is called the stitch width…Used low values and high values, but still could see a seam at certain angles and larger values just made that ridge wider. I wouldn’t really mind the ridge so much except it runs along the seam, meaning it is straight, and thus artificial-looking as a hill.
What surprised me about this was that the HMs I was using for the terrains were all part of the same larger HM that I just cut into tiles. The edges should have matched without the use of any stitching, but, even though they didn’t, they were pretty close.
I was debating sending you the project file to see if you could see if it is something I’m not doing right, but thought I’d just take a poke at one huge terrain.
Due to the way the terrain normals work, in order to create a true seamless connection between terrains, the two terrain pixels from either edge have to be the exact same value. In other words there’s a 2-terrain pixel strip that’s the same height. However this shouldn’t be noticeable in practice, once the terrain is textured (and in particular with vegetation). Also, when changing the stitch width, make sure you undo before trying a different stitch width; otherwise the operation is being applied on top of the previous operation, which doesn’t really help.
–Eric
Yeah. I was using undo a lot. 
Fortunately, when push comes to shove, if I go the stitching route, the only time you see more than one terrain at a time is from a zoomed-out overhead perspective. As long as my lighting is done from directly above (which it will be), it really shouldn’t be that noticeable.