Memtest86+ stalls on a pair of Thinkpad T420 laptops if either has two memory modules installed.

In one of the two T420's the modules are completely identical and made by the same manufacturer.
In the other T420 the memory modules have the same specifications but were made by two different
manufacturers, neither the same as the modules in the first T420. Memtest86+ stalls after a partial
first pass in either T420 when two memory modules (all are 4GB each) are in place, but Memtest86+
runs OK if I remove either T420's Slot (1) memory module. Memtest86+ identifies both memory modules
in either T420 as to maker, specifications, and size.

Is there an enabling setting in the T420 CMOS settings ? A jumper anywhere ?