Is it possible to call from one sim card and use data (internet 3g) from other sim card at same time?
Dude...Remember that Phone has only ONE radio module. Hence only 1 SIM card can work at a time.
Some BB, Nokia, Samsung etc phones have dual Radios. They have DUOS feature that is DUal SIM with S/By. Here also only 1 SIM work at a time but there are two radios - Once could be GSM & Other CDMA. BB 9550, 9850 etc have this dual mode.
Then Samsung has some models where both Radios are always ON. BUt that affects Battery life.
So only alternative for you is to us Call Fwding feature on your inactive SIM so you get all voice calls on your Data SIM & still use Cheap data plan
Hope this helps