How does GS3 dual SIM adapter work?
Unless your phone have two SIM card slots, support as you require is not possible. The market has adapters which goes into the existing SIM card slot of phone and will help you to switch between SIMs after shutdown.  The ONLY advantage of using this instead of swapping SIMs by conventional way is to keep both SIMs together with phone and reduce wear and tear on the SIM slot.

Samsung and other manufactures have dual SIM phones which have two SIM slots and the phone can be configured accordingly for usage. Having said that, watch out for terminologies like Dual Active, Dual Standby. Unfortunately confusion prevails even in manufacturer's web sites. So, to be sure try the behaviour before you buy.

You can read about these in the Wikipedia.