The second app is to eliminate the potential for mid summer crab germ after R1 is gone. It also lends significantly to summer anual broadleaf weed control. Pre-M's do their best job on small seeds. Most summer anuals are bigger seeds (lots of stored energy). Late germinating Crabs are so small at the first frost, consumers don't see them & they don't produce seed heads. If barrier from first app is gone 12 weeks after April 1st then between July & August, you have the potential for some dinner plate sized crabs. Just like last year.
No matter the seed blend used, most of the ESTABLISHING grass types in the real world of commercial Lawn Care are Perennial Ryegrass (so use good varieties). Due to the size of the seed & the tremendous vigor they exhibit, Ryes are notoriously good at establishing in the presence of Dimension & Pre-M. Especially at the heavy seed rate most guys use in the most damaged areas (6lbs Rye/M+). So even if the 2nd pre app. is a little late, it will be tailing off enough after 6 weeks to see some result. But watch the label for the LEGAL "when to reseed after treatment" statement.
Hope this helps.
Barricade has been rumored to cause crop reestablishment problems after repeated use. It died a painful death in AG for this reason. Rate would have a lot to do with this. I can't find any evidence that this has occurred in turf though. So turf rates might lessen the problem.