Well, you could use Javascript to set a hidden form field appropriately, or you could have an option value like "Mod:2" and have PHP separate it at the server....
But neither seem like sensible ideas to me: what happens if someone puts together a bogus form and you get "Mod:1"? More sensible would be to have only one value sent from the form, and have the server look up what the other value should be.
I am of course assuming that if "Mod" is selected, then "2" is always selected. But if the number is different, then it would need to be a different form field. If you don't want people selecting the group number themselves, then you'll probably have to go for the Javascript option; of course, you'll still have to check in case someone sends you a fake form submission.