I thought that I would add some data points about multiple loads at Serve. I also had trouble in the beginning for trying two $500 as others have confirmed. However, I have had no trouble doing $500 and then a different value to a second card (e.g. $250). I did it successfully at 2 stores last week.
Also though dealing with a OV where the funds are locked up. Figured that I would go ahead and wait 7-8 days before acting crazy about it. LOL.