Q. Do you report all the income in one state?
A. Technically, no. But, see a similar question at https://ttlc.intuit.com/community/state-taxes/discussion/re-woking-remotely-for-a-less-than-a-month-...
Q. If you split how do you split?
A. You split the income based on the time worked in each state.
Q. Does it matter where the office is?
A. No.
Q. If you go into the office once a week does that matter?
A. No.
I don't think IL and IN are reciprocal.
Reference: https://remote.com/blog/state-tax-reciprocity-agreements
