I'll add some random, unsubstantiated, wild speculation, pulled out of a, umm, well, let me guess that: When the Rams first arranged for walkthroughs at a
high school, they probably thought they would need the Cowboys facility in Oxnard for a proper workout. Then after seeing it, they decided that they could get a full morning workout at the HS facility, close to where they are staying, without having to travel to and from the Cowboys camp, saving time.
That and there was no way that a Wednesday morning Rams only practice could create the type of attendance (and the PR boosting buzz/optics) that occurred on Monday.