IIRC, the original 610S was rated at a nominal 6 ohms impedance. Not sure about the 610L.
If the 610L is a 6 ohm cabinet, then two of them would present a 3 ohm load. The Concert heads were rated for 4 ohm minimum, so two of them would be flirting with disaster. Some of the later, blackface Concert amps may have been rated for 2 ohms, but at least one person has done the math and concluded that a 2 ohm rating was optimistic, at best, but it might be okay at 3 ohms.
On the back of your Concert amp, near the speaker output jacks, should be some lettering saying something like "4 Ohm Minimum Load". Look for that, then respect it. Too low impedance is a good way to let the magic smoke out of the transistors.