Both SMTD launcher scripts for 7 and 8 use the line
deadlinePath = systemTools.getEnvVariable( "DEADLINE_PATH" )
to get the location of the DeadlineCommandBG.exe, which is then used to ask where the Repository is.
So if you change the DEADLINE_PATH environment variable, AND restart 3ds Max, I would expect both scripts to open whatever the DEADLINE_PATH is pointing at.
Note that the entries in the Customize UI dialog you show are both just simple synchronization scripts that connect to the Repository returned by DeadlineCommandBG.exe, copy all scripts from the \Submission\3dsMax\Main\ folder to the local Scripts folder, and then launch the real SMTD. So the Deadline 7 sync script with a DEADLINE_PATH pointing at 8 will launch 8, and the Deadline 8 script with a DEADLINE_PATH pointing at 7 will sync and launch 7…
There might be minor differences between the 7 and 8 launchers, but they are mostly identical. It is the content of the Repository that implements the actual SMTD you see.
Did you remember to restart 3ds Max when you changed the DEADLINE_PATH env. variable?
If you want to be sneaky, you can hard-code the location of your Deadline Binaries in the SMTD Launchers so they don’t go to ask for the DEADLINE_PATH. You can open the Submit Max To Deadline 7 MacroScript, find the line I posted above, and replace it with
local deadlinePath = @"C:\Program Files\Thinkbox\Deadline7\bin"
Save the file, press CTRL+E to reevaluate.
Then open the MacroScript for Submit Max To Deadline 8 and replace the same line with
local deadlinePath = @"C:\Program Files\Thinkbox\Deadline8\bin"
Save that script, and press CTRL+E to re-evaluate.
Now if you select the Deadline 7 item from a toolbar or menu, it should go looking for the DeadlineCommand of Deadline 7, while the Deadline 8 one will launch the DeadlineCommand of Deadline 8.
But only do this if you are comfortable messing with scripts