This is still a problem in 9.3.5 - it worked in 8.5.5.
If one credential is used for a recipient list and another for attachements I get this error:
Recipient file did not exist.
No recipients were specified.
It works if only one credential is used in the task.
You asked for screenshots:
1: Recipients are listed in a file on a VisualCron network drive:
2: Attachements are fetched on another host via an UNC path and other credentials: