I learned that 'modal'verbs can be contained in one sentence only once. But when I should express two auxiliary verbs in one sentence like 'You can may get this money.', How can we express the same meaning with one modal verb?
you can do something = you have the ability to do it
you may do something = you have license, permission, or freedom to do it
This would not make good sense:
You have the ability to have the freedom to do something.
Having no restrictions imposed on you is not an ability of yours. It is a condition external to you.
When the traffic light turns green, you may cross the street.
If you are able to walk, you can walk across the street.
You can may get this money.
The sentence is ungrammatical; you don't use two modal auxiliaries together in front of a verb. You should use either can or may.
You can get this money = You are able to get this money. It may mean "You are allowed to get this money".
You may get this money = It is possible for you to get this money. It may mean "You are allowed to get this money".
However, "can" is more commonly used than "may" for giving permission.