On Tue, 13 Oct 2020 12:44:53 -0700 (PDT),
wrote:

On Tuesday, 13 October 2020 20:11:32 UTC+1, Dave Liquorice wrote:
Requires an additional wire to get it in parallel with the existing
bell. ISTR that running extra wires was not an option, though not
discounted in the orginal post.

Ding-dongs (which don't have an interrupter contact) should be able to be run in series as long as the bell voltages add up to the power supply voltage.

Understood.

As long as you replace all the batteries at the same time, two similar ding-dongs with batteries in each would equal 8 cells in series with 2 chimes.

Check.

Trembler bells of similar type can be run in series provided that only one has interrupter contacts, which act for all the bells. The interrupter on all the other bells needs to be shorted.

Noted (this isn't such).

Alternatively, I could rewire one unit to be just the bell (takes me
back to my BT and 706 days g) and replace the batteries in the main
unit with 3 x 18650's. ;-)

Cheers, T i m