Initially, can i use 21mg nicotine patch in addition to nicotine gum as needed for withdrawal symptoms ?

Would say no: The 21 mg patch is already a high dose, so adding nicotine gum (or smoking/using tobacco) at the same time increases the risk of a reaction to the nicotine, which is a powerful drug. If you have withdrawal symptoms despite the high dose patch, you likely have a strong addiction/tolerance as part of the problem. But i would only add more nicotine if instructed by your physician.

Nicotin: You need to talk to your doctor (who knows your medical history). Combination therapy is sometimes used (REF: https://www.quitplan.com/how-to-quit/quit-medications/combinations.html). 21 mg is already the highest of the doses.