BRB under the section for Special Rules, first dialogue box. A model cannot gain the benefit of a special rule more than once. However, the effects of multiple *different* special rules are cumulative.
Since the formation special rule that confers the +1BS to all drones in the army would be the same special rule as granted by having a second formation it does not stack, as the model cannot benefit from the same special rule more than once.
So, yes, it absolutely is true.
I still think it's cool as a gun line idea, regardless of whether or not you get +1 or +2 BS. The only downside is the cost, unless you're crazy like me and have literally dozens of Crisis Suit team kits with 6 drones each. Intercepting, split-firing, twin-linked, jet pack Firewarriors, with built-in (if you want) marker light support. Their Ld still sucks, though. Good thing you can JSJ out of the way of assaults.