Just be clear, are you trying to use both plugins from one interface at the same time?
(By entering the post edit URL into Microthemer via the ‘View’ menu and then triggering the Live Editor)
With the above setup, I noticed that it’s not possible click on SO page elements without triggering the SO interface lightbox. Unlike with Beaver Builder, MT doesn’t support concurrent editing via one screen with Site Origin’s page builder. But I’d be happy to add this to the list of page builders to support in that way.
Just to confirm, can you select elements on pages built with Site Origin’s page builder when editing the published page directly in MT, rather than via the SO page builder screen nested inside Microthemer?