UPDATE: This appears to have to be done in one match!(Thanks noodlz05)
Originally, it worked in separate matches, but not any longer.
This would have to be a mode where there is a neutral flag or both teams have flags like Encounter or Standard Battle. (Thanks DeadMartin)
My best recommendation is doing this in T1 matches. I love the T1 Cunningham--it has a great combination of speed, and extreme hitting power. the tough part will be getting to the enemy capture zone. Once there however, you stand an excellent chance of taking out two tanks depending which ones you are up against. Make sure you know what spots to aim for against the heavily armored T1's!
Sorry about my previous video I uploaded of when I got the achievement--I don't know how it got deleted, as it should've been saved! When I get a chance, I'm going to try to record another one of 'getting' the achievement in T1's.