[LazyPup]

Based upon all the information that has been presented I would suspect that the gage on the second boiler is about 8 to 10psi out of calibration.

Consider the facts:
If the boilers are on separate loops, and further assuming that the feedwater check valve on the larger boiler is allowing some back flow to the smaller boiler, at best the pressure in the smaller boiler could only go to equilibrium with the larger boiler. There is no possible way that it could cause a 10psi increase in the smaller boiler.

If the boilers are on a common loop the pressure would always remain equal in each boiler.

It was also previously stated that when the larger boiler is firing at 22psi the smaller boiler pressure goes in excess of 30psi. A hydronic boiler T&P valve is factory set to discharge at the maximum allowable working pressure of 30psi. On the other hand, even though your gage is indicating in excess of 30psi, there is no mention of any water being discharged from the safety valve. This would lead me to conclude that the problem is in the gage.

In addition, both boilers should have a "high pressure cutoff switch", which is set at the maximum working pressure of 30psi. If the pressure in the smaller boiler was in fact exceeding 30psi the high pressure cutoff would prevent the burner from firing.

I would suspect that if you were to remove the gage on the smaller boiler you will find that it is reading 8-10psi instead of zero when laying on the bench.