mma fight. [joking, i'm being smart]
it depends on the gods in question. if by 'the horned god' you mean The Sage God/Lord as worshiped in Wicca, while he has many qualities, invoking him with another peaceful deity, or a god of the forest, sun, hunt, protection, or other similar qualities, nothing should happen.
if you mean horned god as in Satan, go with a god of mischief or destruction. [Loki or Seth for example] calling upon two polar opposites can result in the spell backfiring and/or the two deities not working together. i've also been told mixing deities from cultures notorious for fighting doesn't end well either. best to stick with the Gods you know and have a connection with than calling on gods that sound cool.