Combine/merge Submaster

The easiest way I can think of would be to create an empty Sub in Blind, say Sub 4 and then recall the data from the Subs you want to merge.

For Example:

[Blind][Sub][4][Enter] - Gets you into editmode for Sub 4
[Group][Sub][2][Recall From][Sub][2][Enter] - Recalls the Data from Sub 2 for all Channels that have Data in it.
[Group][Sub][3][Recall From][Sub][3][Enter] - Does the same for Sub 3.

You could also copy Sub 2 to Sub 4 and then just recall the data from Sub 3. Whatever suits you best.
If there's any conflicting data stored in both Subs, the data last recalled takes precedence of course.