It's probably better to do one. For me, personally, and many other people - one of your related texts will probbaly seem stronger because we're basically under the impression that they'll only ask for two. I'm using only one since i've developed better points for that one related. Plus, if you were to write on ur P.T, then R.T, P.T, R.T, P.T (this is all i could do in 40 mins) then you're probably going to write one idea on each related, but I don't know could be a good thing I suppose?