You literally can 
(Rite/Skill tome showcase, two Kai sabres).
(the patchnote where this was added/stated)
and as far as i’ve seen, its only been implied to work when you use two of the same weapon; IE, you’d need to have two patrimonies to do this.
Patrimony Weapon Stacking Dilemma
effort
2.2
5
quality
2.8
5
reasonability
2.857142857142857
14
(Rite/Skill tome showcase, two Kai sabres).