Reading through your questions, I see two things. Opportunity and fear. First off you have two solid clients and they are generating income, but I see a fear of not only the business development side, but of all the details of starting and running the business. You're letting this stop you from moving forward. Honestly, off the basic writeup you provided, I would not be looking to make things too complicated. Starting a partnership is easy, you determine your startup costs (Overhead, Variable Costs, Salaries) for likely 6 months add a buffer depending on the stability of you revenue source. Once you have this, that's the investment capital required to start the business. Form the corporation, award shares based on the investment capital from the partners, and start working.
Valuation isn't necessary at this point. A service business with no revenue isn't worth much, the intellectual property won't value out to an investor, and the company wouldn't have assets.
If you can afford to live off your agency income, take the money from consulting, invest in the business to grow. Find a business development partner, or employee and get going. Once its stable, leave the agency to run your own company.
Obviously there is a lot more details that can't be covered here, but hopefully this will give you a little to think about. If you'd like to set up a call, I'd be more than happy to oblige.
Don't let fear hold you back, you've proven you have the skills to generate results for your clients.