Did you have an IP agreement or contribution? The problem is that customer lists usually fall into trade secret category and once shared, it is hard to surrender (see leap-frog doctrine). There are 3 possible negotiating postures
a) as asset you brought as intangible partnership contribution, you get to keep all changes so there needs to be a non-compete agreement where not-approach anyone on client list for a fixed period of time (depends on jurisdiction on what is normal)
b) treat as compensatable item, the partner pays a fair market value for continued access to the list
c) as joint property if it has been significantly expanded during course of business dealings, then the trade secret becomes part of the firms asset and you both take a copy or split the nameslist. Recreating the idea under a different name however may come down to unfair competition in some jurisdictions or not dealing in good faith.
Breaking up a partnership is not easy as there may be conmingled assets. There are ways to resolve this using either Russian roulette or Texan shootout. It comes down to how amicable the dissolution is and whether partners are prepared to be professional adults about it.