This is an interesting concept for any of those have a preponderance for getting all of training out of the way in one almighty session each day.
Most athletes have a programme of endurance, sprint/anaerobic and resistance training. Often to get it all in in a given week, coaches rightly schedule several similar or different types of sessions each day. The programme could just plonk one session straight after each other, get it done why not? Get home in time for lunch and get your feet up for the afternoon.
What this study indicates nicely is that if you mix up the signals of exercise intensity the subsequent adaptation will also be mixed. At least there will be a blurring of stimuli, such that one signal would over-ride or dampen the signal of the other. In high performance sports where you are attempting to develop physical qualities to their maximum/optimum, at least in proportion to the effort put in.
What you should be avoiding is doing some training from which you don't get an effect. In that instance, the effort would be wasted or certainly not fully utilised.
The take home message is often that if you mix it up, you will likely not get back quite what you wanted from either training session. If you go to a concert, you will often leave with the song that effected you most going around your head. Let the sounds you sing in training resonate for the full effect