Possibly the solution, if practical, is to combine the scripts. You can then coordinate access to the I2C port. If that is not practical you need to push that management function into a daemon and have your scripts access the daemon to get work done. Without a single "manager" that can serialize access to the I2C buss you are in a world of hurt - as you have already discovered.

Note that often you will need to manage atomic transactions consisting of a series of writes and reads to get work done. So a manager that just provides atomic read or write of blocks of data probably won't cut it. That may mean the manager needs more domain knowledge than is appropriate for a generic black box or has a complicated API.

That said, if you combine your common module idea with a lock file you can probably solve the problem without needing to go the daemon route.