I do it naturally but when changing gear- You should always take some pressure off the pedals to enable the chain to change gear cleanly. That chain under pressure is taught along the top of the chain between the cassette and chain rings. Lots of pressure- as on the tandem- and if it does change it will not be smoothly or quietly. Up hill on the MTB and whether it is up or down and there will be some graunching.- if I time it wrong

And that is when sitting down. I still get the occasional jump on the pedals if I don't time it right but to do it standing up with more pressure than normal is asking for chain or ring damage.

The pro's might do it but they ride bikes a lot more than I do- and they don't pay for the broken bits. They also have a spare bike not too far behind them if they do break things 80 miles into a 150 mile ride.