You can mount two dials to a single base, but I don't see the point. If you position the dial on the axis of the wrist pin, the rocking effect will be virtually nothing. Remember to zero the dial on the deck, swing it over the piston and take your measurement, and then check for zero on the deck again. Moving the base may affect the settings of the dial, so its good practice to make sure nothing moved after the measure.
I was referring to piston rock when using a depth mic, since you'll be pressing the base down fairly hard to keep it flat, and probably off center due to stampings in the top of the piston.
http://www.jegs.com/p/B-B/B-B-Perfor...46426/10002/-1
That is another tool thats handy for doing piston protrusion or TDC location.
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'98 3500 2wd, 24v, 5 spd, p-pumped
'98 2500 4wd, 12v, 6 spd, VP'd
'02 VW Jetta, 5 spd.
My p-pump conversion pics