1. Will a twelve volt transformer run two doorbells?
2. I want to run, bury, a doorbell circuit(see #1) and a telephone line to my detached garage. How deep should they be buried and can they be ran in the same pvc conduit?

Oh, is there any type of marking tape that can be placed above the buried conduit for future diggers?

Does the chime require 16 volts?
If the long run is too far, it could activate a relay in chime box to give full power, especially on long runs.
I would use twisted pair for the underground run with phone line, separate cable.

My answers follows your questions.

1. Will a twelve volt transformer run two doorbells?

Most doorbells are 16 volts, just something to check. The doorbells voltage need to match the transformer voltage, first.

Voltage is not the only issue to match. Transformers also come rated in VA, which stands Volt-Amps, which is AC Watts or power. The transformer needs to be rated a VA that is high enough to handle both bells.

If each bell for example draws 5 VA, then the transformer needs to be rated at least 10 VA.

You can review various signalling transformers, bells, chimes here:
Edwards Signaling - Home Page - (http://www.edwards-signals.com/)


2. I want to run, bury, a doorbell circuit(see #1) and a telephone line to my detatched garage. How deep should they be buried and can they be ran in the same pvc conduit?

If residential, the 18 inch deep is the minimum depth. Yes, these two systems can be in the same conduit.


Oh, is there any type of marking tape that can be placed above the buried conduit for future diggers?

There are many types of warning tape available for the purpose of burying above underground utilities, here is an example to get you started:

Carlton Industries, LP - Underground Warning Tape (http://www.carltonusa.com/c-35-underground-warning-tape.aspx)