There's nothing special about common.js, it's just copy-pasted to the top of both the edittime and runtime script files. So if you have function FooBar() { ... } in both edittime and runtime, cut and paste it to the common file and it should be available in both.
Edit: oh, but those functions end up in the global namespace. It would be best to create your own namespace in common.js, so it doesn't conflict with any other plugins, e.g. in common.js:
var MyObjectCommon = {};
MyObjectCommon.FooBar = function () { ... }; // etc
then in both edittime and runtime you call MyObjectCommon.FooBar(), and it should avoid any name collisions.