No, the energy coming out of the turbine does not have the same energy as when it goes in. The compressed air does work on the turbine by decompressing: this means that the pressure downstream from the turbine is less than the pressure upstream from it.
The reason you don't get more energy when you put them in series, is that you've still only got the same energy being input - the energy in the compressed air. You can't get more energy out than you put in.
The mechanism that prevents the energy out being proportional to the number of turbines in series, is as follows.
If you've got just one turbine, then the upstream pressure is the pressure from the compressor or compressed storage, and the downstream pressure is about equal to ambient air pressure.
Now, if you've got two turbines in series, then the upstream pressure on the first turbine is still equal to the compressor pressure; and the downstream pressure from the last turbine is still about equal to ambient air pressure. And that means that the air pressure between the two turbines is somewhere between the two, which means that the air has done less work on the first turbine, and less work on the second turbine.
All in all, the two turbines combined had the potential to do no more work than one turbine on its own, because the pressure gradient from start to end is the same: except now, that work is spread across two turbines instead of one, so you've got twice the losses, and hence in reality less work is done by two turbines than one.
Note that this applies because the two turbines have same kind of work applied to them by the same kind of energy. Combined-cycle gas turbines get more work out of two turbines in series than one, by burning fuel to create an expansionary force that drives the first turbines; the heat in that exhaust air is then used to heat water, creating steam, which then drives a second turbine. This means that the second turbine does not have to compromise the pressure downstream from the first turbine.