Administrative Assistant Merging Calls for Someone Else

Wondering if anybody has any expertise on Zoom Phone features for someone who is a delegate for a professional and thus, manages his incoming calls.  Is there a way to MERGE two calls on someone else's lines?  

Example:

1. Caller 1 calls Person A and is on the line with that caller

2. Caller 2 calls Person A's line and Assistant answers the call on his line.  Caller 2 wants to be merged into the first call.

Is there away for Assistant to merge Caller 2 into the existing call between Caller 1 and Person A?  We know how to merge calls coming into our own lines.  This question is specifically for managing someone else's lines through delegation/monitoring.

Thank you.