That can be done using World Composition. Each landscape will have to be in it’s own sublevel, but they count as the same landscape and can be edited at the same time. Andrew Hurley talked about it on a Live Training stream earlier this year. Here’s a timestamp to the part of the stream where he talks about it, but I’d watch the whole thing if you’re interested in World Composition. https://youtu.be/FrcNClb6KUI?t=47m40s
If you don’t want them to be in separate levels, then the only other way I know would be to add components like ZomboJoe said by going to the Landscape tab > Manage > Landscape Editor > and choose Add from the dropdown. However, adding too many components to a single landscape could be bad for performance.