When copying and pasting several things in Emacs, I've always done it like this - alternating:

• copy A
• paste A
• copy B
• paste B

It only clicked for me today that it's easier to do the copies together, as long as you paste in the reverse order:

• copy A
• copy B
• paste B
• paste A

I've always known that when you copy or cut ("kill" in the Emacs lingo) some text in Emacs, that the selection goes onto a stack, called the kill-ring (since it actually loops around). You can then paste ("yank") and immediately cycle through previous items on the stack with yank-pop (M-y). Very handy if you get side-tracked and want something you copied a while back.