Eric, I agree that with a water cooled motor and hot gasses and hot water coming out the same tail pipes, that you will see some condensation in the exhaust, especially under certain conditions of dew point etc. The thing to watch though, is a sign of extra "steam" on one motor or another.
Water pressure in the cooling system can force droplets through the gasket somewhere, or through a pinhole in the exhaust manifold when the motor is running. When water droplets hit the hot gas flow of the motor exhaust, this causes steam, sometimes a lot of steam. If one motor is producing some significant steam and the other is running without any, this is cause for alarm.
Water can get in through a crack in the block water passages to the cylinder; crack in the cylinder head; freeze crack in the exhaust manifold; rust hole in the motor water passage; rust hole in the exhaust manifold; gasket failure between coolant passage and exhaust passage gasket; gasket failure at the intake manifold.
A crack somewhere at the cylinder head or a head gasket leak may show up on the plugs somehow. If you get a hole that allows water so drip in over time when the motor is shut off, you could end up with enough water draining into the cylinder, draining back from a cooling passage to cause damage when you try to start the motor.
As you said, some condensation is normal.
When a thermostat gets the motor warm and the hot water is allowed to start to flow out the system, you can get some condensation too. The only way I know how to "read" the appearance of the exhaust, is to look at one side or another for an abnormal situation.
Roy
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