Yes, I hook them up in series. I can't remember if I did that the first time, but I'm pretty sure I did...At 600ma you should get about .6*2.2 watts. Also Far Red are not going to be as bright as regular red.
You said your driver is 4-12V so it could have an issue with single LED.
Did you test by connecting both LED together in series and powering, +ve of one LED connected to -ve of other LED. In series it should try to pull around 4.4Volts
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But are you saying there should be no problem powering these leds with that driver? Cause the driver runs the cxa1304 just fine, so it seems to be good. If there is no obvious problem with this setup I'll try the two untested 730nm leds.
I might consider hooking up one of the 730nm with one cxa1304 if the sum voltage fits better within the 4-12v range. Which I think it should, though I haven't checked the Vf of the cxa1304 @ 600mA yet.
Wait... Could the issue be that the one of the two 730nm leds that doesn't respond, has connection issues, lets the current straight through itself and the resulting load is only one led, which the driver can't run properly given it is only 2.2V?
I'll try the pulsing 730nm in series with a cxa1304.
EDIT: Turns out the cxa1304 is 9.75Vf @ 600mA... Combined with the 2.2 of the 730nm it's a little too close to 12V, right?