AGM Dual Battery - Connecting only the negative terminal in parallel - battery leakage

I recently attempted to replace the starting battery (Battery #1) in a temporary dual parallel AGM battery setup. I had removed the starting battery and in the interim was relying on the deep cycle accessory battery (Battery # 2) for starting as well. (Note: I do plan to eventually add a battery isolator for a true starting/accessory battery setup.)

In the course of installing the new Battery #1, I inadvertently neglected to attached the positive cable between Battery #1 and Battery #2. Specifically, the positive terminal of Battery #1 was only connected to the alternator. However, the negative terminal of Battery #1 was connected to the negative terminal of the Battery #2, which in turn was grounded to the engine. The positive terminal of Battery #2 was still connected to the starter, which enabled me to start the car. Shortly after starting, I noticed liquid leaking out of the vent hole of new Battery #1. I immediately disconnected it believing Battery #1 was defective I didn’t realize at the time that I forgot to connect the positive side of the Battery #1 to the positive side of Battery #2 – duh! (See picture/diagram)

So my question: Was it user/installer error that caused the leaking (possibly from overcharging) or was it simply a defective battery?