Can i add two extenders on one camera ?

Sure, that is called stacking. Carl the truth is half right...it will significantly lower the usable maximum aperture of the lens while the extenders are attached, but that does not mean you must shoot in bright light, it just means you will need to compensate by either increasing ISO, or making your shutter speed longer. In fact, one of the most common shots that uses stacked extenders is of the moon at night. Sounds like Carl the truth does not really understand how a DSLR works...he should probably stop answering questions like this until he figures it out...