I ran a side project for about 12 months as a subsidiary of my main business, but as soon as I brought a co-founder on board for it (as the side project grew),

I ran a side project for about 12 months as a subsidiary of my main business, but as soon as I brought a co-founder on board for it (as the side project grew), I filed for creating a separate company (Australian Pty Ltd - even though my co-founder was US based). That way, I could formally list both of us as directors and shareholders on it. Much cleaner legally (and tax wise) too, plus it made it easier to set up a separate bank account etc.