Having two entrances to my front patio and only one door bell transmitter invariably meant that all visitors would try use the 'other' entrance. After having a quick search for the model number I found the specs online which described the HPM D642 model as
factory preset to one of over 19,000 unique channels
After purchasing a second set I got to hacking. For the transmitter, the back off the unit unclips revealing two screws holding in the board. Once those are out, the board is free. Given it's sub $10 price it couldn't have been all that complicated, and as expected, it was just 9 solder links pulling an input high, low or leaving it floating. This gives us 3 ^ 9 or 19,683 different combinations; sounds about right!
To make things simple, I numbered each of the links then described each as L (left), N (neutral) or R (right). The above was coded as NLLNRNNRN
Next, opening up the receiver required removing the battery cover and then a single screw revealing the circuit. At first glance this board only had 8 links, however near by was the missing 9th. Although as you can see from the picture below, in my case this link was neutral (empty).
Now you'll notice that in the overlay I have numbered the transmitter 1 to 9 going from top to bottom, however on the receiver it is numbered 1 to 8 bottom to top with the 9th over to the side. Also note, that the 9th link on the receiver board is back to front compared to the others! That is to say that a link on the transmitter that went 'left' would be joined 'right' on the receiver. A final caution is that if a 'left' link is used for the 9th link on the receiver, it may instead be linked to the pin 'above' (underneath the 'N') instead of to the pad to the right (L). The two are both on the same track so it would seem they have found that easier to join in the factory than pad to pad.
The final step was to simply use the trusty solder sucker to remove any of the links that didn't match my old unit then add any that were missing. Reinstall into their respective cases, install a battery (or three), and bingo! Two transmitters ringing two receivers :)
You can buy a set that has one transmitter and two receivers, however for me I needed the opposite! Oh well.. I guess now I can have a receiver in different rooms too.
Just remember to make sure you've cleaned away any trace of solder between the links or you'll end up with a short circuit across the battery! I would recommend using a multi-meter to make sure you don't have continuity between the + and - terminals! Would be a good idea to check you've got a good joint between each link too. Also note that you're almost certainly voiding your warranty by modifying your unit and are doing so at your own risk.