Exactly what you should do depends on how many tapes you've got in the environment that's going away, and/or how much money you want to throw at the problem.
Symantec supports three methods of merging domains, although only one is really a merger :
1) Hire Symantec Consulting to merge them.
2) You manually reimport all tapes/images from domain2 into domain1. Add all domain2 clients to domain1.
3) Add all clients from domain2 to domain1. Leave domain2 up and running until all images & tapes have expired. Shutdown domain2.
Option #1 takes about a week, mostly for initial config documentation and catalog consistency checks, but costs a few grand.
Option #2 costs no money (your time is free, isn't it ?) but it can take up to several hours per tape (depending on how full it is, tape size, # of files in the images, etc). If there are enough tapes to import, or you have limited drive time available to do the imports, this can make your 'merger' last months.
If you've got a reasonable number of tapes to import, or your retention periods are short, then doing it yourself is the way to go. If you've got multi-year retentions or hundreds of tapes, you should probably look at hiring Consulting to do this for you.
Good luck !
