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Both new (Group size 24) AGM batteries were charged six days ago. Today, I tried to start with switch on #1. Sounded like the battery was nearly dead. The same with #2. Switched to "All" and the starter motor came alive vigorously and within a second, the brand new 50HP E-TEC started. My 26X (and Perko switch) is thirteen years old. The switch is tight, no play and all appears to be in place. The E-TECH was installed late July, launched August 1st and started perfectly on #1. Thereafter, when starting with #2 it would sound nearly dead. Today, as I mentioned, #1 and #2 don't seem to deliver the power yet "All" does.

Why would #1 suddenly sound weak after working properly and #2 would never deliver the power? Did the E-TECH dealer have to touch the Perko switch during the installation?

Check the terminal connections for tightness (and corrosion); any that might have been touched or disturbed by anybody lately. Might be at the batteries themselves, at the switch, or at the engine underneath the cowling.

The contact points on the switch can be a source of a poor connection, even though it feels “tight” – they might need inspection and cleaning, especially if exposed to a salt water environment. Sometimes spinning the switch and working it with contact spray will give you a temporary fix, which indicates further disassembly and cleanup will be needed for the long-term.

BTW – when checking connections that are secured by screwed fittings (such as nuts on the battery) twist the cable connectors gently counter-clockwise as you do. If they are already a bit loose, they will immediately become looser, making the poor connection obvious. On the other hand, if you were to grab and twist in a clockwise direction, it could possibly just tighten it and mask the loosness.

Were the batteries charged with an external charger, and then installed in the boat?

As Seahouse said, it is likely a corroded or loose connection.
Disconnect both + and - cables from both batteries. Then take an ohmmeter and measure the resistance along each leg of the path to and through the A/B switch.

Thanks!
My gut tells me it's the Perko switch, possibly corrosion. New E-TECH motor known for its charging alternator, new batteries kept at full charge (trickle charge) with an outside charger while on board and connected, and tight cable connections. I'm returning today and will start in the "All" position.

I had a very similar sounding problem which turned out to be the ground wire to the battery being made loose by a recent wiring mod I'd performed. Make sure the cables to the perko-switch are "first" on the battery screw terminals, and that your motor is "first" on the perko-switch screw terminals, because it's the highest amp draw on the boat. Then have any other connectors screwed down tightly above that.

Thanks. I'll check it out soon. Windy today, so no boating.