You can always keep this thing bootstrapped. If you reach a point you need an extra pair of hands, you can evaluate if it makes sense to hire someone, and if debt or equity financing is necessary. But if you're not at that point, there's no reason to take that on.
A key thing to remember is being invested in is a means to an end. Ultimately it requires giving up a level of control, or taking on a level of debt; it's not simply 'a good thing to do'.
So even without considering the downsides of a customer as a shareholder, you may want to avoid doing it just because you don't know how the money will advance your goals.
Given the growth you're having organically on just two people and not that much time, I'd try to keep on your own a little bit more rather than searching for investors. That will take time, and hiring more people will also take even more time. Maybe it's worth it to put that time into getting new clients, knowing the market (i.e., what's your upper limit, what companies you can't reach, etc) and maybe growing revenue so when you do take investors and hire more people you're closer to breaking even.