All the CAM software, including ViewMate allows panelizing boards and even creating panels containing more than one design.
There are two application notes that describe how this can be done:ftp://ftp.pentalogix.com/Documents/AppNotes/Panelizing%20Gerber%20Data.pdf
They are a bit out of date and reference an older product (ViewMasterEZ) but one can do the same steps in the current software.
Finally, a simple overview follows below:
Import the data for your first board. Let's say that occupies layers 1 to 9. Note the StackUp order of the files. Place these files where you want them on the panel. You can create multiple copies if you wish.
Activate layer number 11. Import the set of files that corresponds to the next board. Make sure the StackUp matches the first set. These files will now occupy layers 11 to 19. Select the data on layers 11 to 19 and move it where you want them to appear on the panel. You can also create multiple copies of this set of data.
When you've positioned the files where you want them, you can select all of the data on 11 to 19 and transfer the data to layers 1 to 9. You now have all of the data for both boards on layers 1 to 9.
When finished, export the data.