I hold passports of both US & Canada. Born in Canada, resided until 20 yrs of age. Moved to US and became naturalized US citizen. My question is: Can I legally LIVE & WORK simultaneously in both countries commuting back and forth on a weekly basis?

As a US citizen, there are no restrictions... you can live and work anywhere in the world as long as you wish... the only condition, you must file tax without fail even you don't have any income in US at all.