Two versions of Acrobat installed on a PC - can the older one be the defaut?

I have a user who has Acrobat DC and Acrobat X installed.  She needs both, but wants X to be the default PDF handler.  The option to make it so is grayed out.  Tried one registry trick but that didn't help.  Acrobat X isn't even an option using Open With.

Is it possible to make X the defaut in this scenario?

Thank you,

MilesFromMO