When I used the arduino multiple stepper testing from the arduino library, the 2 motors is working fine. But when I used the Universal Gcode Sender, motor is not responding at all. Everything is working fine from Arduino to Grbl.
Please tell us which motors you have - the infomation you have given isn't helpful, I'm afraid, give
the full part number, datasheet, website, anything more than "5 - 12 Vdc" which means nothing
(stepper motors are rated by current and winding resistance, not voltage).