Are you running them both of the same TE line? If you split the line
too close to the varios, they will interfere with each other causing
"jumpiness"

This is wrong. Neither the 302 nor the B40 use flask(s) so there is no
flow and therefore no interference. You can connect them togeather
with no risk of interference.

One reason why the 302 needle can be way off is if there is sunlight
on the display when you power up. The sunlight can disturb the optical
sensors that detect the needle. You can see this when you power off -
if the needle does not return to 0 then it was incorrectly detected at
power up. make sure the 302 display is in shadow when you power up (eg
using your hand).

Peter