I think planning two states in one trip would be too much as road connectivity may not be as good as you would want it it to be. Plus, hilly terrain will ensure that you take lot more time that you had envisaged at the beginning of the road trip. Meghalaya is a real beauty, and is waiting to be explored. Same is the case with Arunachal Pradesh - but road connectivity in this state is extremely low.
Ya while traveling through these destination, a highway will come, on the left side it's Meghalaya and on the other side of the road it's Assam. that place is just amazing, you can actually see mountains starting from the other half of the highway
U should definitely try TAWANG- ARUNACHAL PRADESH and MEGHALAYA. There are many places in ARUNACHAL PRADESH to explore, but the problem there is the Distance. Almost every other major locations from Tawang is approx 13 to >14 hrs away. I suggest Tawang because it can be visited en-route from MEGHALAYA or Assam as well, or vice versa. I am planning a SOLO trip to SIKKIM, TAWANG, & MEGHALAYA this October 2019. Will be doing, most probably a bike journey in SIKKIM, not sure of TAWANG. But will do Tawang too if I get company. All the best for your plans.