What AC-to-DC adapter would I need to power two of these battery (3V) powered door contacts at once?
A 3V DC adapter would be appropriate. Wired in parallel. Make sure it is a regulated 3V adapter (measure the voltage of the adapter without a load, if it's 3V, you're fine), many old/cheaper AC-DC converters are unregulated, and will produce a much higher voltage when not used with the equipment they are designed for.