I have, against my will, been unable to answer until now. I want to thank for your attention and inform that I consider this matter solved.
I missed an existing stanza for Void in the Grub menu, due to a layout in Lubuntu which differs from any one I knew. As for Lubuntu itself I have come to the conclusion that Grub places one only menu entry for partitions with identical kernel and initramfs. If this is absurd I would appreciate hearing about it. This understanding was based first on the observation that os-poberinfoms that it has found both (identical) kernels and second on the fact that by editing the only menu stanza for Lubuntu and replacing the "root+=" parameter with the value for the other partition with the other version of the same kernel I was able to swiftly boot in that other partition
