Have you ever seen maggots migrating(crawling)...flies transporting them---I have!
No, I have never seen that. I have never known of blow flies exhibiting that behavior. Of course, just because it isn't known, doesn't mean you did not witness it. If that's what you saw, it would be very intriguing to many entomologists, I'm sure.
But are you sure you did not see a parasitic Tachinid fly giving live birth to maggots. Some species of Tachinids do that, and they look very much like house or blow flies.
Do you remember exactly what you witnessed?