If the PSU has two separate 12V outputs, connecting them together is a bad idea. One of the outputs is likely to regulate to a slightly higher voltage than the other, and will end up taking nearly all the load. If you are using RAMPS, then I suggest you use one output to power the heated bed (the 11A circuit), and the other output to power everything else (the 5A circuit).
You can't measure the "current of a power supply", all you can measure is how much current some load takes when you connect it to the power supply.
You can't measure the "current of a power supply", all you can measure is how much current some load takes when you connect it to the power supply.