Unsure how to use media only screen codes

With these media only screen codes..

@media only screen and (max-device-width: 320px){

@media only screen and (max-device-width: 640px){

Do i need to put the codes in between both the 320px code as well as the 640px code? Or can i just put all the codes underneath both of them?

What i mean is i currently have it set like this..

@media only screen and (max-device-width: 320px){

CODE HERE

@media only screen and (max-device-width: 640px){

SAME CODE HERE

But i'm wondering whether i could put the code underneath both @media screen codes, like the following..

@media only screen and (max-device-width: 320px){

@media only screen and (max-device-width: 640px){

CODE HERE

Hope that makes sense!

To understand how to write media queries please follow the articles :

https://css-tricks.com/logic-in-media-queries/

https://www.w3schools.com/css/css3_mediaqueries_ex.asp

Hope this helps.