The mid-field Customs facility operates (approximately) 6:00AM-8:30PM. That covers the early morning UA,SA and ET arrivals doing the "split".

For UA international arrivals (which haven't pre-cleared US Customs) and all their Star Alliance partners that also do the "split", connecting (TRANSFER on the signs) passengers use the mid-field Customs facility regardless of what airline/destination is their connection flight - assuming all flights are on the same ticket.