You do need two executors. I'm assuming from the way in which you've worded your post that you are single, (otherwise, people normally name their partner as their executor, and then two others to act in default).
So, assuming that you are single, you need to have two people to act. This is because, in the very unlikely event that you died before your son is old enough to take control of your money, then a trust would automatically arise, and two trustees would need to administer the trust.
So far as guardianship is concerned, it's fine to appoint two people. I'm a will writer and often suggest that my clients specify who they would prefer the child(ren) to live with, but also put in a backup in case the first person isn't able to act, for example if they emigrate. As an alternative, you can appoint two people and not specify which you'd prefer to have the children day to day, and leave them to decide between them who would be better placed. That would depend, for example, on how old the child was.
You can, if you are concerned about avoiding any misunderstanding or ill-feeling, explain your reasons for choosing a particular guardian.
Depending on whether your son's father has Parental Responsibility, it may be more appropriate to specify that the guardian(s) will only act if the father can't or is unwilling to care for your son.
I know you have an appointment sorted out, OP, but if anyone else is still putting off making their will, I have a paid-for advert over on Mumsnet Classifieds (Small Business) titled "5* Will Writing Service Recommended by Mumsnetters".