[Neil]

I don't think you can do that anyway. You use one dimmer switch and one normal 2 way switch.

I take it you originally had two 2way switches. You leave one switch as it was and replace the other with the 2 way dimmer. You wire up the new one as the old one was with the wires going in L1 L2 and common just the same.

Dimmers on stairs is not a good idea. If someone sets the bottom dimmer so low that the light is off you will fall down stairs in the dark trying to get to the switch to turn the light back up again.

If you had two dimmers they would both be trying to dim the light they would not be able to function correctly as you expect.