You can get the role of Wicket Keeper under Law 40:
https://www.lords.org/mcc/laws-of-cricket/laws/law-40-the-wicket-keeper/
Now come to the next question, when the match starts if the primary wicket keeper bowl the first over, the secondary wicket keeper will start keeping, and except that the primary wicket keeper will play the role of wicket keeping. Choosing of secondary wicket keeper is not compulsory. But if anyone choose the secondary wicket keeper, he will get the facility to place his primary wicket keeper in the bowling line up. As well as secondary wicket keeper can also bowl when the primary wicket keeper will play his role of wicket keeping.
