I just put my Polar 720 on the bar next to it and use the interval function for time only (without a heart rate strap). It also records altitude which the PT is not capable of. The difficulty is for intervals on the TT bike where there's no room so the watch has to be on my wrist. This time of year with long sleeves it sometimes gets covered by my cuff.
There is always enough space for one, two or three computers on a handlebar
Especially when it is for a trainingride. The costs of a second CPU isn't skyhigh compared to the costs of one SL 2.4 hub.
But I think that more information visible about my average power output when riding a 20 minute L4 interval outdoors is a welcome addition.