Each timer has an outlet that energises for the safelight. Use this to energise the coil of a relay for each timer. The contacts should be wired to an outlet daisy chain style (in series) so both have to be energised for the safelight to be on. The safelight plugs into this setup. I'd put the relays in a project box with clearly marked cords for the timer connections and a power cord for the safelight to plug in the wall and an outlet to plug the safelight in to. Heres a rough sketch. I could put together a working circuit in minutes if I had the parts on hand.
It could easily be set up for a normally closed setup to actively turn off the circuit using the enlarger plug or with a normally open to use the safelight plug if available.
Gary, similar to an earlier post with SPST relays as an example. Either way, your method also looks sound to me. I think the key factor here is the inline or serial use of relays to guarantee either timer will indirectly (via relay) create an open situation in the main circuit.
I'll rig something up in the coming months and let you guys know if I blew anything up.
BTW, given a typical US 120vac/15a circuit, I'd presume we wouldn't be drawing 15 amps through the control side of the relay, as that would be nuts. Is the solenoid (or similar) on the coil side of the relay the only limiting factor of current draw from it's source - or does it use some form of resistor?