It depends on

a) the gauge of the wire
b) How much each card drains through the pci-e slot

First one is pretty easy, just look at whats written on the yellow wire going into your molex chain probably something like AWG 20 or AWG 18, then consult this chart: http://www.engineeringtoolbox.com/wire-gauges-d_419.html (go by the gauge on your wire and look at the amps noted on single core). Multiply the number by 12 and you have the max amount of watts this wire was designed for.

Second one is tougher but you can assume the maximum any card will drain from the PCI-e slot is 75W since that's the max allowed in the specification.

If you're just gonna test it then most electrical wires are rated for up to 75C, as long as it stays under that at full load, its fine.