Wal-Mart sells mechanical temp gauges (made by Equius) for around $10.
They come with a good selection of fittings and mine is very accurate.
You should be able to install a "T" piece fitting that will accept
both your electric gauge and the mechanical one simultaniously. Try
vairying the the engine conditions (NEVER rev a motor above 2000
r.p.m. in neutral). The two gauges should follow each other closely.
Now, test the other motor. All done for around $15.

Just be careful not to kink the temp gauge line, it's filled with
liquid (alcohol, I think). Ideally, this test should be performed
under normal operating conditions.

Another method, but do this at your own risk, is to ground out the
wire to the sender unit via a selection of various resistors. For
example, if a 500 Ohm resistor takes the gauge to half, and a 200 Ohm
takes it to one quarter, it MUST do the same on the other gauge. Next,
remove the sender units and put them in a pot of warm water for a bit.
Using a digital multimeter, record the Ohms readings of both. Heat the
water a little more and repeat, all the way up to near boiling. I'm
guessing the readings should be within about 5% of each other.
Remember to make the readings quickly, for accuracy. Problem is, if
they're way out, which one is faulty???? This is why I preffer a
mechanical gauge over an electric gauge whenever possible, even for
Volts (joke!). Unfortunately, the pivoting and steering system of a
boat does not lend itself to these gauges, unless they're mounted on
the motor directly (another temporary option).

Okay, since I have a few minutes, and it were my boat, here's what I'd
do (at your own risk of course)

1) Replace the other impellor (water pump).
2) Ensure that all the seals between the pump and the head are good.
You could even be losing water pressure/volume from a leaking
seal or joint somewhere along that pipe (been there, done that).
3) Remove both thermostats, and leave housings open. BRIEFLY run
engines at idle and a little above. Both "fountains" should be
similar in volume and pressure (use drums, not ear muffs, for a
true simulation.)
4) Verify that both thermostats are working similarly, by using the
pot on a stove method.
5) Verify that both temperature gauges are accurate/reliable.
(also check for and clean any corrosion on any terminals).

Basically, you start at the beginning (pump) and logically eliminate
any discrepencies between the 2 motors.

I would also suggest getting a manual for your motor. They are
invaluable.

Good Luck.

I really appreciate your time on this! I just got the manual via UPS
and my wife ordered one of the temperature laser gun things. And I
will be starting through your suggestions starting with the water pump
in the "hot" engine when I get back home this weekend.

Thanks!
Eldon