Actually, for variable valve-timing, two camshafts per cylinder head are necessary, one for the intake port and one for the exhaust. If done at all, then the timing for the intake and exhaust valves is varied separately. How this can also be done in an engine with only one cam per cylinder head (OHC), is shown below in the Mahle company's Cam-in-Cam video below.
Basically, there are in fact two shafts, however, one is inside the other. In order that the inside shaft can turn certain cam-throws on the outside shaft, pins and slots on the outside shaft are necessary. The advantage is, more space than a single cam needs, is not necessary. That means, that there are hardly any cylinder head alterations necessary.
Of course, this system can also be applied to twin-overhead cams. As you can see in the video below, apart from the simple cam, other forms are also possible. There may even be two cams responsible for the same valve, which rotating in opposite directions, significantly lengthens their opening times.
To shed some light on the advantages, e.g., in the case of the last mentioned point: Assuming a direct fuel injection, the engine constructor tries to leave the exhaust valves, at full load, open for as long as possible. This way, he can avoid the swirling of anything inhibiting. Residual gas is flushed out anyway by having too much fresh-gas. The oxygen content is increased and the temperature is lowered.
These are favourable conditions for as high a compression as possible, which goes hand in hand with a high efficiency. However, if these longer exhaust valve opening times were to be maintained in other load ranges, or even only in the lower load range, the exhaust valves of various cylinders could be open at the same time, which could lead to turbulences and a mutual inflow.
This is why, e.g., to combat turbo-lags, one needs a an exhaust flow to the gas turbo-charger, which is as free as possible, thus saving oneself the need of an additional compressor. It's quite astonishing, that the Mahle company has had initial successes on the exhaust-side, whereby, it's generally the intake-side that's considered as far as any conceivable innovations are concerned.
This doesn't mean that the cam-in-cam system has no advantages for the intakes, particularly as it can be combined with the customary adjustment system on the camshaft's drive-wheel. In this case, even the Atkinson-principle is possible: One selects a geometric compression, which is actually too high and then, if knocking threatens, the intake valves(s) are left open long enough to allow air to flow back into the intake system.
The above described advantages of the effective through-flow of the combustion chamber with a low residual gas development, are also made possible by the very early opening of the intake valve(s). Despite the fact that more valve overlap is caused, there is no overlapping of the exhaust gases of individual cylinders. When studying the cam-in-cam system, one is forced to admit, that all the possible advantages have not yet been recognised. 04/14