I would strongly urge against mucking around with a reservation which is actually ticketed and especially where the mucking involves another carrier. That is a prescription for a problem on DFW-LAS.
AA or US run 12 non-stops/day on that route and you can pick one which is convenient. As C-W-S notes, separate tickets do not matter. AA will rebook without penalty as a matter of policy and US does it as a matter of customer service.
The sole difference in process is that if you are on one ticket and BA is late, it is BA which will rebook. If you are late to DFW on two tickets, it is the operating carrier of the onward segment, AA or US, which will handle.