Hi Anne and all:
Well, if you are using GNU / Linux and open each Presentation From the file browser I'm almost sure that you can keep both open and then select the slides from one Presentation to the other (Selecting all that you want to into the other with copy-paste). I know that it is better using an option insert from the own program but if it isn't able or finding, this other would be solve your problem (If it exist yet).
I think that using windows the process is the same, I guess.
I hope this help,
Regards,
Jorge Rodríguez