If I leave an unattached Infernal Shade on the ground (via Lingering Flames) and a Pyre Golem that already has an Infernal Shade attached walks over to it, the old shade is overwritten by the new Shade. I was told this was a bug and should report it here.
You can tell it has the new shade rather than the old shade because I’ve taken the “Subjugation” node and the damage starts over when the new shade becomes attached.
Another bug that I found is likely only visual. I leave an Infernal Shade on the ground (via Lingering Flames) and when my Pyre Golem walks over to it and picks up the shade, the icon next to my health globe indicates that I have two active shades, which is not true. The icon fixes itself if I cast Infernal Shade directly onto the Golem or have it pick up another.