Gene Kearns writes:

If you have some sort of objective scientific explanation reaching
beyond having seen a car battery explode and consequently becoming
afraid of them.... I'd like to hear it.

The explanation is simply that batteries are more or less mismatched. See
my earlier posts. As long as there is a mismatch, you cannot get twice the
watt-hours out of a pair of batteries in parallel versus a single. One
will always load the other. There is also a risk of severe overload if a
cell should short; this risk is simply not present in a series arrangement;
thus my recommendation to insert fusible links in each parallel branch.
Aside from the output degradation, and the shorted-cell risk, yes, you can
put well-matched batteries in parallel. It is common in certain
applications as a compromise. If series is possible, then use series.