In blowouts, I find that if you put in different players as starter/backup the minute will work out almost evenly. I had 2 players get exactly 24 minutes each in this scenerio yesterday and two other positions produced 25-23 and 28-20 splits.
To get the minute distribution you want above, you're better off putting the same player as the starter/backup and put the 2nd guy as the reserve (3rd spot). That usually gives me about 36-12 or 38-10 split in a blowout.