I think someone said something about rolling each set twice. Anyway, here is another idea.
Any time there are more than X dice being rolled (you decide X for yourself, but I would say at least 4 dice [you can’t average 1 roll, and it would get absurd if you roll twice for 2 dice, perhaps absurd for 3]), then the person rolling them rolls them two times, then takes the Average number of hits between the two rolls. In case of decimal, round in the direction of the first roll.
For example, I am attacking with 3 inf and 3 tanks against you with 3 inf. I would roll twice, lets say the first roll I get 2 hits, and the second roll I get 5 hits. The average of this would be 7/2= 3.5 hits. Because the first roll was lower, you round down, so that is 3 hits now. You would not get to roll twice because you are only rolling 3 dice.
Or another example, I attack with 3 inf 2 art, against you with 3 inf. I roll twice and get 2 hits the first time, and 1 hit the second time. The average is 1.5, but you round up because the first roll was higher. You again would not roll twice because you have less than X=4 dice to roll. You get lucky and roll 3 hits. Now it is 2 art against 1 inf. Neither of us would roll twice because we are now both less than 4.
Or another way to do this, would be to use a certain number of “roll twice for an average” chips / tokens. Lets say you get 5 total for each team, and are allowed to purchase more for 4 ipcs each. You may use one of them up to do exactly what I just described above.