It works just fine.
I preffer brewing them separately at first:
Beforehand find your caapi dosage by drinking caapi-only starting at maybe 20g and increase dosage in subsequent tries until you find dose where you definitely feel effects, like for example bit of tracers, affected headspace and maybe light sensitivity, but not overwhelming. Then next time you can take that same dosage with no more than 3g mimosa first, because mimosa can be very potent.
Traditionally they were never used together because mimosa and caapi come from geographically separated areas, one is from amazon deep rainforest, the other is from dry `caatinga` some thousand of kms away. But pharmacologically it definitely works. Ive had good blissful experiences, as well as difficult experiences (and both in same experiences too), but a lot of other factors obviously come into play.
Good luck.