Boxcryptor on Windows does not do any synchronization, this is solely done by the installed sync-clients themselves.
To achieve something like that, you will need to come up with a solution "outside" of Boxcryptor and then apply the Boxcryptor solution "on top" of that, e.g. have the resulting folder added as a Boxcryptor location.
Here is something that may just work:
I noticed that you can sync any folder to Google Drive Backup and Sync via the context-menu. Thus, you could try to do this on a folder which is within the OneDrive location.
If this works out, this folder is then synced by both cloud providers and also can be accessed via the Boxcryptor drive (
X:\OneDrive\<folder also synced with Google Drive>)