I have an MKR1010 plugged into MKR2UNO.
The 5-volt pin reads 5 volts when power is supplied via the USB port.
If I plug a 5-volt supply power to the MKR2UNO power connector, then the 5-volt pin reads 3.73 volts and not the 5 volts expected.
I would expect that a 5-volt pin would be 5 volts at all times?
The real question is: How do I get a 5-volts to a 5-volt sensor using this combination. Do I need to feed the power via the USB port and not the power port on the MKR2UNO?
From looking at the schematic of the MKR2UNO it's hard to tell. There is a voltage regulator showing a connection to VIN, but the diagram shows VIN connected to the supply side of the regulator. Something is not right here.
The schematic diagram is crap, also that of the MKR1000 
Multiple different lines labeld VIN, no 3.3V regulator...
The power connector IMO requires a higher voltage, as input to the 7805. This also is how the power connector of the Uno works.
Is IC1 really a 7805, or a regulator for 3.3V as required by the MKR1000?
Connect 5V power to the USB port or, at your risk, to the 5V pin.
jcfowlerca:
I have an MKR1010 plugged into MKR2UNO.The 5-volt pin reads 5 volts when power is supplied via the USB port.
If I plug a 5-volt supply power to the MKR2UNO power connector, then the 5-volt pin reads 3.73 volts and not the 5 volts expected.
I would expect that a 5-volt pin would be 5 volts at all times?
the barrel jack is input for the regulator. Input Voltage (recommended) 7-12V
MKR2UNO Adapter — Arduino Official Store Tech Spec tab