Sub-questions / comments
a.1) Straight TTL Serial isn't going to help you with 10m cable... (Why does a clock have 10m data cable?)
b.1) You have to disable Serial on the non-reset Arduino
b.2) Why can't you just make that a serial command?
b.3) How can an alarm clock overflow??????!!!!!
TIP
If you want more useful help, post the schematic and tell a bit more about it. We can help you NOT to reinvent the wheel 