Help... Adding a second temp gauge

There are several issues here none of them can be good
1....The original gauge is heavily damped so as not to show the pulses of the voltage limiter/ regulator.
2....The Bosch appears it might be a different design known as a "bridge" circuit, completely different
(3 terminals one of them grounded)
3.....It is very unlikely that the two meters use the same sender RANGE and therefore would never be accurate
4....As noted above, interaction between the two may make both inaccurate

You have to realize that the original gauge is a form of simple ohmeter working on current. When you change that current flow by adding a second load to the sender you are likely to upset the applecart. "Ohms Law." You have created a parallel path with more current flowing through the sender. For all I know it might be enough to damage the sender or at least affect it's accuracy because of the added current flow