Ive joined the 2 5vs together on a few and never had a issue. The ECU is fecked anyway so it makes no diffence should it overload it. Ive never seen one overload as its only a signal. The wirings under no load.
I have often thought about 12v-5v converters or something like this which can be screwed to the body
http://www.ebay.co.uk/itm/Voltage-Regulator-7805-fixed-5V-5Volt-output-1A-1Amp-/251526682738?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item3a90289872
I have often thought about 12v-5v converters or something like this which can be screwed to the body
http://www.ebay.co.uk/itm/Voltage-Regulator-7805-fixed-5V-5Volt-output-1A-1Amp-/251526682738?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item3a90289872