Question: Between a mains step down transformer and a SMPS, which would be safer for a bench power supply? I cannot look inside the transformer in either case to judge the insulation quality. But, if the insulation does fail there would be 230VAC at the secondary for the mains transformer. And, maybe 325VDC at the output of the SMPS (is that possible)? I can visually inspect the SMPS PCB to check if there is proper clearance, presence of MOVs, thermistors, fuses etc, but of course cannot see inside the transformer.

So the question is, which is easier to make safer (or screw up)? Which is easier to figure out when new, if failure will be catastrophic in future? Can SMPS transformers fail due to failing mosfets etc? I'm guessing that if a mains transformer works without issues when new (including the problem of interwinding capacitance), I can expect it to work for at least a decade without issues, right? Assuming everything will be fused properly and fan cooled.

It's one thing to get a shock from inadvertently touching bad wiring, and quite another to be exposed while clutching the wires in my hands. So I am concerned somewhat about safety. Of course, an RCD/RCCB should probably be put in, but I don't have that yet.

[PS: I'm planning to put a buck converter and then a linear regulator to drop and adjust the final output voltage, be it the SMPS, or the mains transformer.]

Answer: 
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> Battery A still had potential of 12V, but the battery B showed 0V. 
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When you connect batteries in series, you have to make sure they are of equal capacity (Ampere hours), equal Voltage and equal wear and age.  

Basically you may only create a series pack with two brand new batteries.

This is important to prevent one battery limiting the entire pack. When discharging with unequal batteries your drain one of the batteries to damaging levels, and with charging you overcharge the other. 

You can buy lead acid battery equalizers that make sure they are both the same voltage during charging and discharging so they wear out at the same rate.  

However, this is not strictly required. But it can be helpful when you have such a heavy load that the differences in internal resistance at high currents have an effect on the balance.

For other battery chemistries, such as lithium batteries, balance is more strictly managed since these batteries are a fire hazard when you (dis)charge them too much.
The battery connected to the positive terminal will drain first every time. The idea that they drain evenly is incorrect. I check my trolling motor batteries for voltage every time I bring my boat back from the lake. The one connected to the positive lead is low and the one connected to the negative lead is 12+V.