Can i use 2 different brand toothpastes? can i use sensodyne whitening toothpaste with colgate toothpaste? i hear the colgate whitening is abrasive.

Yes: Yes you can but it does not make much sense to me. Pick one and stay with it.

Whitening t'paste: Any t'paste labeled "whitening" is abrasive to scrape the stain off the surfaces of your teeth. But if your teeth have darkened because of intrinsic, not surface, stain, the color won't change very much. Talk to your Dentist about the proper way to whiten your teeth.