I did not know that such a gizmo exists. Until I received an email from a Chinese company called Betterpart.

It looks like there are more companies offering a Dual SIM card adapter. The device allows to manage two GSM Sim cards in one mobile phone. This means users can switch between to numbers or networks. More and more people have two mobiles. One for work and one for private. If the Dual SIM cards work those people can make things easier by just needing to use one phone.
Betterpart has detailed instructions on their site how the Dualsim adapter works. It incorporates to cut your Sim Cards a bit.