Hey Lonnie. Is this a one time activity, or something you are going to repeat during your project?
I would say manual copy/paste is probably going to be the most straightforward, then if there is VBA associated with those symbols or display then copy/paste that too.
However, if you are talking about repeatedly doing this activity with multiple pairs of files then you would probably write a VBA script to copy the symbols (read definition from one display, add to another display), change the tag references (Symbol.SetTagName) and move over any VBA using VBA Extensibility. Placement of the symbols is also fairly straightforward, you just got to get used to the top of the display being 15000 and the left -15000 when moving the symbols in VBA, but it sounds like you would be moving them relative to some symbols from the template display anyway.
Thanks Rhys for the great answers. This is going to be an automated merge based on an AF attribute and will be part of a workflow. It sounds like it is possible by writing some scripting in the "master" template, where I will go and copy/paste the content of the second pdi file and change what I need to.
I just wanted to make sure it was possible before I step off the deep end