I'll post pictures then.

Also, please notice that the temp delta between the two HDD when under the positive effect of the fan, is CONSTANTLY 1 C degree, no exception, for the last three days. Meaning, literally (and a bit surprisingly) EVERY time one HDD temp increased by 1 C degree, the other HDD temp did so too.

When without FAN, instead, the WD was, as you may remember, between 3 and 10 C degrees cooler, temp delta being much more variable over time.

This seems to suggest that the two HDD produce different amount of heat as operating conditions change, and such temp difference is not proportional among the two. BUT, the fan seems to largely offset whatever difference there is, keeping them basically at the same temperature (almost: 1 C degree in favor of the WD, which however is BELOW the Seagate, so that alone could explain the 1 C difference).

Cool (!) phenomenon to observe, I have to say...