Do I need to buy different cards for NTool and EasyBox Mtk?

No. You need to buy only one smartcard for both tools. But you cant launch both tools at the same time with one smartcard. You need to close NTool before launching EasyBox Mtk and vice versa.

Do NTool and EasyBox Mtk use different credits?

No. NTool and EasyBox Mtk use the same credits for their paid functions.

Do I need an Internet connection to use EasyBox Mtk?

You dont need an Internet connection for the majority of functions of the EasyBox Mtk. But some functions cost some amount of credits. To use these functions you do need internet connection.