tach misreading - showing wrong rpm's

I agree, should be able to put another teach in parrelle with the other.
is that possible? currently i have a speedhut tach. On a MSD6HEMI ignition box. I am not sure if so but it is possible that the Tach is showing ie 6000 rpm but actually the engine is reving 7000 rpm?

Can I put an Autometer tach parallel on the MSD Tach port to check that?