As for joining the two companies, I assume both have a value. Either get an outside valuation or agree on a valuation method and join together based on current values.
If that doesn't work, yes you can negotiate to have a value of the new entity based on your work so far and your upcoming work. All that said, I would seriously question going into a partnership with people you haven't known for a solid amount of time. If you do decide to go into a partnership, remember this: Most partnerships DO NOT work out. Even marriages fail at a rate of 50% these days. Seriously look at how you can both work together for a period of time and both get the benefits, but don't officially partner with each other until you are really sure that forming a company together is the best thing to do.
All the Best,
Brad Sugars is the founder and chairman of ActionCOACH. As an entrepreneur, author and business coach, he has owned and operated more than two dozen companies including his main company, ActionCOACH, which has more than 1,000 offices in 34 countries.