Connecting Together 2 or more terrains prefabs

I've tried and read many things but haven't found the answer... Please help me.

I have 2 prefabricated terrains, they from different tools. I need to connect these 2 terrains ... But I can't ... Do you have any options to do this? I'm a developer, I'm not a molder, I hate doing this, but I have no choice, I'm doing it myself ...

What I need, left side, mountain terrain, with snow, right side, a plain ... I need the plain to connect with the mountain, the left edge of the plain, connect with the right edge with mountains ...

I see a different option using C #, I don't think I need it, does the unit have no tools to do this? The unit editor connects correctly if I use adding simple terrain, with a mirror, I look wonderful, but I need to create the terrain, and I hate it and I suck at it ... And in the asset store have some tools (like gaia), but so much expensive (for my country), and other free, like terrains toolkit, that I have used to create the right side terrain, but their not join different terrains.

PS: I've tried too do this manually, but how the terrains are side by side, the tools from unity do not work only in the selected terrain, they work in all area below the brush, and rise the mountains too, then i need to rise only the plain side.

Can someone help me?

Thank you and sorry for disturbing your ...

PS2: Sorry, but i cant put images here, i am using google drive, and this forum do not accept. The link are public, and I've tested with others persons and opened, but here, they not work... sorry...