Might work, might not!
In a perfect setting, you can do this. In most real world settings, this can lead to hums in your IEM's, the PA, etc due to not sharing a common ground, or by being on opposite electrical phases.
What you need to use is a transformerized mic/line splitter for each microphone or line level source you are splitting between your IEm mixer and the house mixer. Something like a Rapco MS-4, or an ART S8 which is an eight channel mic splitter in the same price range. The Rapco likely has higher quality transformers and it is made in the US.
Each input channel has a ground lift switch (to cure the hums you are creating by tying devices together from different AC circuits or phases or due to nasty electricity. The Art has mic/line pad per channel as well. Actually not a bad piece.
You might get your "budget" splitters to work in a rehearsal environment, but it is at gigs where you will have the most issues. Basically, hear me now or thank me later. But using straight splits will lead to grief at some point. Take that $60 and save it on up for the solution not the band-aid.
Particularly if you are dropping custom mold cash, spend the money on something of this nature and make it worth having spent the extra dollars on a workable splitter. Good luck!