You might try a transformer with a higher VA rating at the same voltage. Sometimes, but not always, paralleling two identical chime transformers can be adequate to boost VA enough.
If that doesn’t work then you may need to come up with a relay with the correct coil voltage to connect the common front door button and isolate the two chimes and their respective transformers from each other. Either that or you would need to develop a double pole single throw pushbutton and connect each chime independently to it’s own transformer.
Maybe just find a chime unit that has auxiliary outputs for another remote chime. I would look at some of those wireless units, maybe you can program more than one to operate from a single remote pushbutton.
