Hi,
It is "technically" possible to stack MM licenses but in your case going with 1 MM-VA-500 might be a better option if you consider any future expansion. You already have two 7205 controllers so theoretically you can have 256 APs with redundancy or 512 APs without redundancy. Adding additional MM licenses in the future will be more costly. Note in both cases, you need to scale your VM with MM-VA-500 specs. For sure, you will also need to add the necessary AP / PEF / RFP licenses as per your requirements. Also, note that in the first option (3X50) you only have 8 extra licenses as you will be using 140 for APs and 2 for controllers.. You don't even have 10% buffer to grow.
Finally, by having an MM, you can benefit from Clustering technology which allows AP and User load-balancing so it is even better than active / active! Check this link for additional details
https://community.arubanetworks.com/t5/Aruba-Solution-Exchange/Clustering-of-Mobility-Controllers/ta-p/282686