It is my current situation that I work in an Illinois based office for an Illinois based company, but I live in Missouri and have also worked several months throughout 2017 at the Missouri branch.
My W-2 only shows Missouri Tax withholding with a Missouri Employer ID, nothing about Illinois.
I should also mention I started work in Missouri before my company transferred me to the Illinois branch.
I realize there is no reciprocity between Illinois and Missouri, and this situation is a bit bizzare, but do I have income tax liability in Illinois? And if so, would I have to try and split my reported income over the year between the two offices?
Illinois (like other states) taxes non-residents on income earned from physically working within its borders. Therefore you do have to file a non-resident return in IL in addition to your home state return in MO. MO can tax ALL your income, including your earnings from IL, but you'll be able to take a credit on your MO return for the taxes paid to IL, so you won't be double-taxed.
This FAQ explains how to do a non-resident state return in TurboTax: https://ttlc.intuit.com/questions/1901271-how-do-i-file-a-nonresident-state-return
Note that you should complete the non-resident state return BEFORE you do your home state return.
Illinois (like other states) taxes non-residents on income earned from physically working within its borders. Therefore you do have to file a non-resident return in IL in addition to your home state return in MO. MO can tax ALL your income, including your earnings from IL, but you'll be able to take a credit on your MO return for the taxes paid to IL, so you won't be double-taxed.
This FAQ explains how to do a non-resident state return in TurboTax: https://ttlc.intuit.com/questions/1901271-how-do-i-file-a-nonresident-state-return
Note that you should complete the non-resident state return BEFORE you do your home state return.