Unless your existing Sim Card change "push the outer plastic away to become nano, otherwise you have to go to maxis to change.
As for using 2 sim cards at any time. You have request it from your telco for additional chargers. Even though same number for both Sims, the settings is different from telco side.

Unless you meant changing your sim card to your other phone. There is a sim card converter/ adapter , that looks something like this http://www.amazon.com/Converter-Adapter-St...r/dp/B00B5KXRZO you can get it at lowyat or similar IT phone gadget shop in Malaysia, its even sold in garage sales trade zone in the forum if you know where to find

pretty much the case , but new sim card these days (the standard sim card) can change from big to small by yourself (just like puzzle , you will know what i meant once you got the sim card) and use adapter.