Forced Reconciliation
I think perhaps the two slaves had been fighting, so their master decided it was time they got a little closeness and together time:
From alt.binaries.pictures.erotica.bondage. It looks amateur to me, probably French going by the martinet, which is still largely a French thing and rare elsewhere.
They could be sharing a double-ended dildo. When the one on top feels the lash of the martinet on her rump, she jolts from the pain and thrusts deeper into the one on the bottom.