This rare formation – an adverbial use of the present participle of severer – is also found in English, where it is equally uncommon. See also severinement.
adv.
jointement et severauntment
1lawjointly and severally, (of two or more persons) with equal liability both as a group and as individuals:
(
1313
)
nos [...] grauntoms de estre tenuz juntement e severauntment [...]
422
severauntment (1313)