You may choose your executors to be the same people as your beneficiaries. There is no legal problem with this. But it may not always be appropriate, e.g. if they do not get on with each other or if you think they would find the task too difficult.
In those circumstances, it may be sensible to appoint someone else to act, such as other family members or friends, or possibly professionals, such as your solicitors.
It is common to appoint substitute executors in your Will, e.g. I appoint my husband as my executor but if he has died before me or is unwilling/unable to act, then I appoint my son and/or daughter to act instead.
Your appointed executor or executors must be 18 years of age or above.