The Mara and the Serengeti are the same ecosystem cut by a national border, which is exactly why combining them works so well on paper and takes real planning to execute well. Wildlife doesn't stop for immigration, but travelers do, and the border crossing itself needs to be built into the itinerary with enough slack that it never feels rushed.
The bigger design question is sequencing. Some seasons favor starting in Kenya and finishing in Tanzania as the migration moves south; other months reverse that logic entirely. Getting this order wrong doesn't ruin a trip, but it does mean paying for game drives in a park the animals have already left.
Velvet Trails plans these routes around where the herds actually are that month, not a fixed template, and we build in a full transition day at the border crossing so nobody's first afternoon in a new country is spent recovering from the drive in.