UA reserves the right to get upset about this if you appear to be intending to violate a fare rule. The most obvious example of this is if you're nesting a ticket that returns to your origin city in order to avoid a minimum stay requirement, and even then you have to be a serial offender for UA to notice. So, if your second ticket were EWR-ORD-EWR, and there was a Saturday-night stay requirement on the ORD-EWR fare so that it was a lot cheaper to do it this way (as opposed to ORD-EWR-ORD / ORD-EWR-ORD), they might care.

The scenario you described is absolutely fine and will create no problems at all.