When you add them separately, make sure you get one that is either significantly bigger or significantly smaller. The reason behind this is, when you add it some temporary fighting will ensue to establish which is going to be the dominant bigger female and the smaller male. if they are the same size, the fighting could get brutal and lethal.
"If your wife ever asks how much something was for your tank, just tell her you got it on sale and you had to have it! She'll understand"