two output signals on pins 28 are connected together.
When either device is selected we cannot obtain a signal
from pin 28 as the other device seems to interfere with
the signal i.e. each IC can only operate wit the other IC's
pin 28 diconnected. Is it possible to link pins 28 on
You can connect the outputs of two AD506 multiplexers together but you must use
the enable pin (EN) to arbitrate between the two devices. Only one mux should
be enabled at any time and you need to take care when switching between the two
muxes that the connection is a break-before-make.
If two muxes are enables at the same time, the two outputs will fight for
control of the combined D line and you'll see interference.