I have conjunctivitis in one eye and the other eye has under gone cataract surgery 2 weeks ago how serious this can be. ?

Potentially: Since you are still in your post op period of your cataract surgery, you should see your surgeon to make sure both eyes are ok. Eye infections are contagious and can spread from one eye to the other. Please call your ophthalmologist to be safe...

Get it checked: Typically common conjunctivitis in the non surgical eye 2 weeks after surgery is not serious. I would suggest having your ophthalmologist examine it non-emergently just to ensure it actually is conjunctivitis and not something more serious.