According to Catholic Doctrine Catholics do not pray to Saints as a deity-thus it would not be considered "shittuf" as they are not joining or raising Saints to the level of the Divine. Rather, according to this site Catholics use them as emmisaries and intermediaries in their requests, and that the saint will argue for you to their deity.
According to this site the difference lies between prayer and worship:
The Difference Between Prayer and Worship
Many non-Catholic Christians believe that it is wrong to pray to the
saints, claiming that our prayers should be directed to God alone.
Some Catholics, responding to this criticism, have argued that we do
not pray to the saints but with them.Both groups, however, are
confusing prayer with worship. True worship (as opposed to veneration
or honor) does indeed belong to God alone, and we should never worship
man or any other creature as we worship God. But while worship may
take the form of prayer, as in the Mass and other liturgies of the
Church, not all prayer is worship. When we pray to the saints, we’re
simply asking them to help us, by praying to God on our behalf, or
thanking them for having already done so.
Seemingly, there is precedence in Judaism for such a concept- albeit radically different than the Christian concept (they are not considered close "friends" and there are no tzaddikim set aside as "charms" for specific needs or requests) this mirrors the practice of asking Tzaddikim to be a "meilitz yosher" or an advocate in heaven.
This idea is mentioned in Gesher HaChaim Volume 2 25:9 in the name of the Zohar Parshas Acharei Mos