What is the plural possessive of Mr?

There is no standard form for the abbreviation Mr. in English.

The plural form of the noun mister is misters.

The plural possessive form is misters'.

For example, the brothers Jim and Jack Jones share an apartment:

• I have the misters' apartment key while they are away.

Normally, when two or more men are referred to by a single term, their names are used, for example:

• I have the Joneses' apartment key while they are away.
• I have the Jones brothers' apartment key while they are away.
• I have Jim and Jack's apartment key while they are away.