Yes, professional bleaching gels can be used with teeth whitening gel but it will NOT lighten crowns and make them whiter, and you could always give your crowns a good polish with the home dental polisher from happysmileuk.com this will make them lighter in colour as it will remove any stains from them.
If you need advice please call Happysmileuk on 0207 298 6117 or please speak to your dentist.