I think "Patroni" is fine. I don't think a plural is used in canon. Not all Latin words in -us are pluralized to -i, but I don't know the rules and for a limerick I'm not sure it's that important.
Needs another syllable in the first line, and the third is awkward...
Two Patroni -- one stag and one doe Protecting a friend and a foe. The danger was dire; Black evaded the fire. Snape's motives? We shall never know.