Willmott Dixon and Circle 33 are working on regeneration and new-build projects worth £100m in east London.
Housing chief operating officer Chris Durkin said the dedicated unit to handle Circle 33 work would create considerable benefits. He said: "Having such a guaranteed workload from Circle 33 has allowed us to do this. We couldn't do it with any other clients yet."
The group will save £850,000 from merging two regional offices which deal with Circle 33.
Willmott Dixon and Circle 33 will choose subcontractors together and plan to share an office.
The group's contracting division has set up a special unit for telecoms group Nortel. It is undertaking two projects worth £50m for the firm.