Wireless Digital Door Chimes are on sale. If I purchased TWO units so that i could have one receiver downstairs and the other upstairs would the bell push unit on the door activate the two receivers or would they be on different wave lenghts.
If both use the same radio frequency/channel it should work.
Ask the salesman and arrange that if it doesn't work you can get refund on one.
Or, if you know someone with basic electronics skills, it is an simple matter to add an activation wire from one sender to the other one with a wire through the door.
My set-up is the same as woofgang's. You can take the battery part anywhere. I take mine into the garden to make sure I hear the bell. Very very useful when I am in the shed.