Hi Stef,
I have the same problem. What I did was to create a new project with the double horizontal resolution and split later. This is a weird method, but it worked for me. The problem is that since the resolution of the clips and the sequence doesn’t match, you won’t be able to preview in real time without render before, so it needs more time.
If anybody knows a way to edit for two screens with the same time line, please post!
all best!