The latter is not necessarily terrible practice given what is available commercially. However the above will statistically double the chance of a premature deployment, increase the opportunity for wiring error 2x to 8x over a single system and leave the system vulnerable to EMI induced errors from two altimeters working close together (depending on the design of the individual altimeters).
I disagree with the logic here. Again putting it in airplane terms (sorry that's my day job), using that logic a 4 engine airplane would have 4 times (or more?) the failure rates of a singe engine airplane. A 4 engine plane has a higher chance of an engine failure causing an in flight shutdown but less of a chance of a forced landing
Would a ground test find a mis-wired altimeter? IF the main and drogue charges were reversed (easy to prevent but still a possibility) the failure mode would be main deployed at apogee, not great but not as bad as having no event at apogee. If I have 2 IDENTICAL altimeters with 2 independent systems I have reduced the chance of;
A single bad battery causing no event
A single bad e-match causing no event
poor packing in a charge holder causing no event
a loose wire causing no event
a failed altimeter causing no event
Arguably I haven't changed the chance of;
shear pins failing to break
failure to turn on altimeter (if you forget 1 you probably would forget two)
incorrect sample hole size causing too early of a deployment (hard to ground test)
You have increased the chance of
a failure in an altimeter causing a premature event
both charges going off at EXACTLY the same time causing airframe failure
All of the failure modes could be reduced by ground testing. I could even make the argument that a singe altimeter with two batteries would get most of the benefits of having 2 altimeters.
I would argue that a wiring DESIGN failure should be detected during ground testing, a wire installation problem, most likely would be the incorrect wiring of the main / drogue, or a loose wire, the loose wire is less likely to cause failure with redundancy. the chance of miswiring the main / drogue, again should be prevented, but could increase in likelyhood by having 2 altimeter. But what is the criticality? Main at apoge better than nothing at apogee.
The third system that is used in aircraft is to give a correlation of information, in other words 2 altimeters that disagree doesn't give much information, having 3, gives the chance to determine what system is giving potentially false information.
A separate system that monitors the other two is standard with 2 altimeters, the ground check is the third check. Different voltages from two batteries, stop what is the bad battery, resistance in charges the same... stuff like that. Having gone over the failure analysis, in my large rockets I use the same charge size for the main and drogue, this eleimates thechance of suing the wrong size charge.
Sorry, putting away soap box, cracking open a beer, sitting back down now...