If i join B.Sc general this year & join dip. in engg. next year & continue both the courses, will both the certificates be valid

No, those will not be valid. you can not enroll for two degrees at a time. If you mention that you have done two degree during same period in your resume it will be considered as invalid. I would suggest you to focus on only one program from some good university like nirma, lovely professional univ

No. One program can only be regular. So please join B Sc (General) from IGNOU and next year you can go for regular Diploma program. Both will be valid. Good luck!