Another thing to check is any shower heads where you can turn off the water flow at the shower head itself.
If you have one of those shower heads, then the flow can go through the shower valve from the cold water supply piping into the hot water supply piping. The result is that you can get cold water coming out when you open either the hot or cold water faucets somewhere else, like the bathroom sink.
If it were simply a blockage in the hot water piping, then you'd have a weak flow of hot water out the affected hot water faucets. If the pressure with which the cold water comes out is as strong out the hot water faucets as it is out the cold water faucets, then I'd agree there's a cross connection somewhere.