I must apologize. I misspoke.
Not a joint venture, but rather a separate company with them as partners. Open up an EC company, under 1 lic and 1 business permit.
The HVAC contactor will hire the EC and pay the new company. Profits will be split accordingly. HVAC contractor saves some money. EC doesn't have to build a company. If they get work from on outside source it's a bonus.
again, my apologies.