Many competitive bodybuilders divide their program into two daily training units during preparation for a competition, which is often referred to as a double split system.
Apparently, they do this to maximize limited energy reserves as a result of strict, low-calorie, low-fat diets.

A group of Finnish exercise physiologists have studied the merits of dividing high-intensity exercise programs into either one or two daily workouts.
In the study, nine strength athletes were in intense training for two separate periods of three weeks each.
During the periods of time, there was high-intensity training, but only in the first two weeks.
In the third week the training volume was drastically reduced both times.

The first phase consisted of just one daily workout, while the second phase split the same program into two daily workouts, like a double-split program.

After two weeks of intensive training, the test subjects' concentration of free testosterone, that is, the testosterone available for use in the cells, fell significantly, although their total testosterone levels remained the same.

The scientists also recorded huge increases in luteinizing hormone (LH), a substance produced in the pituitary gland that regulates testosterone production in the body, and increases in cortisol, a stress hormone that promotes catabolic, or degradative, conditions in muscles.

After two days of rest, the levels of both free testosterone and LH normalized.

With two daily training units, however, a clear advantage over daily training in terms of increasing strength was proven.

The decrease in free or available testosterone was explained as a consequence of the increased demands on two high-intensity training units per day.

The scientists also stated that the elevated cortisol levels were signs of an induced catabolic state.

The scientists could not determine the exact reason for the obvious superiority of the program with two daily training units, but they suspected increased activation of muscles by the nervous system as the cause.

Over time, however, the increased cortisol levels could lower testosterone levels in drug-free bodybuilders, which would result in reduced progress.

We can therefore assume that the double split system may be very effective for short periods of time.