Literally simulque recumbentibus means "and reclining at the same time". Recumbentibus is the ablative plural of recumbens, which is the present active participle of recumbere, to recline. Here the participle is being used in absence of a noun, so it's more like "and those who were also then reclining" - i.e. the people eating with Herod.
Aha, indeed the word does appear in the book you pointed out, in English, meaning a knockout blow. It's in a play called Mankind, which dates from c. 1475. Quite an obscure word!
And I found this from the parallel translations of the Latin Vulgate Bible:
6:22 Cumque introisset filia ipsius Herodiadis, et saltasset, et placuisset Herodi, simulque recumbentibus; rex ait puellæ: Pete a me quod vis, et dabo tibi:
6:22 And when the daughter of the same Herodias had entered, and danced, and pleased Herod, along with those who were at table with him, the king said to the girl, "Request from me whatever you want, and I will give it to you."
My curiosity is piqued. The most information I can find on recumbentibus is that it was spelled wrong in the 2006 National Spelling Bee and this quotation by J.R. Newman:
the advantage of inflicting upon an assailant a recumbentibus
A quick search yielded that the most prominent J.R. Newman is a 20th century mathematician James Roy Newman, who wrote on formal logic and worked peripherally with the Manhattan Project. I'd guess that his use of the word recumbentibus refers to the atomic bomb. I'm tempted to say he coined the word himself, but given his figurative use of it and the etymology oblique to the meaning, I think it more likely that the word is archaic. The thot plickens.