Yeah, it is allowed in all forms of dynamic apnea. They do competitions in both 25m and 50m pools. The turn around time is so close between the lengths as to make it irrelevant. This sort of practice is very good for all kinds of freediving as it allows you to focus on your mind and body without any distraction. This in turn allows you to develop a very solid technique rapidly: further relaxation and slowing of heart rate; maximal stroke dirft; timing of stroke combination; mental focus and control; consistency in stroke etc. The dive becomes a kind of "meditation" or zen practice.
A zen garden. Notice any parallels?