Per the below found on ATTA's website the it may be possible at some point in the future to legally cross between the Triangle and the Serengeti.
I have mixed feelings as this would be a great way to see both parks and allow access (from the Kaogatende airstrip) to other parks in Tanzania. I am however a bit concerned that it may cause an influx of min-busses from Tanzania into the Triangle during the migration period. (If they allow drivers from one country to guide in the other).
I'm already planning an itinerary should the boarder ever actually be opened...
- fly into Nairobi, stay in the Mara Triangle and Serengeti for a few days; flyi to Lake Manyara from Kogatende (Manyara NP and a crater visit); drive down to Arusha NP, fly from KIA back to JKIA.