The first flash is probably for light metering (Canon call their version e-TTL, Nikon iTTL if you want to read about how it works), but the external flash doesn't know that of course.
You should be able to get an external optical trigger that can handle this (a hotshoe with a tripod thread on the bottom). Optical triggers are cheap so this would be a viable solution. Slave triggers that will do what you want apparently exist: "For example, I have a very-inexpensive underwater optical slave that can be set to count various numbers of pre-flash pulses before arming for the "real" flash" (photocentric.com).
Here's a list (dpanswers.com) of some models that apparently ignore a preflash, or attempt to smartly ignore a string of preflashes. The brand reviewed there is Wein (models HS-XL D, PN-XL D, L8 D and XL8 D -- might be out of date now).
Apparently the Seagull SYK-5 Trigger can be set to ignore red-eye reduction flashes -- it might work for you (Amazon in the hope that the link won't die too soon).
Some Yongnuo external flashes can be set to an optical slave mode (davidpartington.com) if you're shopping for a new flash.