Yes, it's possible to use the planner in landscape mode. The options below are the apps we know about (so let us know if you find another). If your app is not listed (e.g GoodNotes), then that app doesn't have the landscape mode functionality.
1
Xodo (all devices)
To turn on landscape mode, go to view settings and choose Facing (2-page) view:
Now rotate your screen and you should see your pages side-by-side
2
Flexcil (Android)
This is possible on both the free and paid versions of Flexcil, but if you're using this app we recommend the paid 'Standard' version (it's a one-time fee).
Open the settings menu and choose these settings to set up 2-page view:
