Two towers... your car could've come straight from Lord of the Rings
Anyway, I don't think it is the optimal option to just add another tower or use a much thicker tower. The problem I've heard is that the tower tends to hit the top mounting lug of the motor plate in a crash. The motor plate is aluminium and placed perpendicular to the tower, so the plate doesn't flex. In other words, when the tower hits the motor plate it prevents a certain percentage of the length of the tower to flex and absorb the impact. Result: It breaks off.
Putting two towers on there just adds a lot of material: It's no guarantee that it is even nearly twice as strong in practice, as the thicker tower flexes less. If you'd land a jump without absorbing it with your knees you'd be in for pain and perhaps injury too... That's exactly what that shock tower is also going through. And I haven't even mentioned that when the shock tower then does become strong enough, it transfers all these loads to the part it is mounted to: the gear casings... I doubt you'd want to break these (even though they are relatively cheap)
The solution ekt mentions is simple and doesn't require any mods to the motor plate or shock tower afaik. I'd give that a shot first and if it keeps breaking often, start viling some material off the lug, increase the offset/spacing, and as a last resort move to a thicker tower.