Dual Sims enable you to hold two Sim cards in one phone! Hold a contract SIM, and a pay as you go sim in one phone, or sim card from different networks. If you travel abroad or call other networks regularly this is an ideal solution to lower prices.
Dual sims work by simply turning your phone on and off. Every time you do it, the phone switches between sim cards. You may need to get your phone unlocked before you can use them.