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If for some reason you find yourself toting around one cellphone and two sim cards at the same time, you might want to look into purchasing a dual sim adapter like the one on the right. The Powerful Dual Card offered by Hong Kong's mobile.brando for $27 lets you switch from one sim to the other without powering your phone off; there's a menu so you can either switch manually or set a timer to do it.

Dual Sim card adapter makes it easy [MobileMag]

Powerful Dual Card []

Cheaper roaming: One phone but two numbers [International Herald Tribune]