From ESRI's FAQ:

Question

Can multiple users edit a file geodatabase at the same time?

Answer

Yes, it is possible to have multiple users edit a file geodatabase at the same time provided that they are editing:

• separate stand-alone feature classes at the root level

OR

• separate feature datasets (this is regarded as a workspace containing one or more feature classes)

OR

• the same feature datasets in the file geodatabase but different feature classes

If these rules are not followed, a schema lock error is received by all users editing the geodatabase, other than the primary user that locked the file geodatabase. If a file geodatabase is not editable at the current time, an appropriate message is received by the client application.

source: https://support.esri.com/en/technical-article/000012031

If I understand it correctly I think you can't work around this and need a real (not a file based) database to do concurrent writes. Something like SDE, Oracle Spatial or PostGIS.

Thanks Niels,

I do have a real (SDE) db at my disposal but it's production. I guess we use gdb as our working dirs, then push to SDE. They don't have identical rules but they do have similar.

Maybe I'll spin up 'working' SDE.

I'm still not sure how this will go though, the same issue might be present. I don't want to tell each child workbench that's running to write to a different db version.

As I do not know the next steps it is hard to say what a possible solution may be.

What is the current workflow? Collect the changes in one gdb and manually check and submit to SDE?

You could dynamically generate a file geodatabase per file and merge the these gdb's into one session gdb.

I guess I COULD produce 80 odd GDBs, just feels cumbersome.

If you go the SDE route I would add a staging table to write the results in. When check is OK then push to production.

Just an informal staging area? polyline_staging?

Yeah, just like the way you used the "staging" file geodatabase.