As an alternative to using two of these DC opto couplers for AC you could use one preceded by a bridge rectifier (Say 4 x 1N4148) or even a single diode as a half wave rectifier. If you also added a capacitor after the rectifier it would get rid of the pulses on the output of the opto coupler. One thing you do not seem to considered is where you want to tap off the existing bell circuit. I expect it to be wired with two core cable from the transformer with the cable going via the bell and finishing at the bell push. The system being discussed is best connected to the bell. One connected to the bell push would have to invert the sense of the signal and one connected near the transformer would have to work by sensing current rather than voltage..
Les.
Les.