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.
By: Lark Moderator / Knowledgeable Mar 02, 2014
Post # 4
Gods are not objects used like spell ingredients such as candles or incense. They are powerful beings with a will and a purpose beyond human concerns. You don't just order them to appear. They show up when and if they wish to do so. Trying to summon a deity just to do a spell is highly disresptful and will likely result in them either not appearing or in them deciding that you need a lesson in respect.
well there is a reason i said what i said and it wasnt to point out the obvious. what i mean is that some underestimate the power of the gods and you question seemed to me that you did. i apologize for the miss communication.