If we just want to offer all leave in one general Paid Time Off (“PTO”) bank can we do that?

You can offer all paid time off in one bank, but it may result in owing departing employees a greater amount for "unused vacation" time.
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There is no problem with offering all paid time off in one bank.  However, while Illinois employers are required to pay out all accrued but unused vacation at termination of employment, they are NOT required to payout all accrued but unused sick time.  Because of this, some employers find it advantageous to break PTO into two separate banks of time.