The two firms will replace Carillion, which was the third partner in the original consortium to secure C2 and C3 contracts in the central section of the project.
Lot C2 will see the construction of the north portal Chiltern tunnels to Brackley, while Lot C3 is for the Brackley to Long Itchington Wood Green tunnel south portal.
Since the two contracts were let the combined value has soared from £1.34bn to £2.48bn.
The new team of contractors will deliver an 80km stretch of HS2 through the countryside, from Wendover in the Chilterns to Leamington Spa.
The route includes 42 km of cuttings up to 30m deep and 27 km of embankments up to 13m high. The firms will also build 70 over-bridges, 15 viaducts, 15 under-bridges and three Green Tunnels with a total length of about 6.5km.
Bam Nuttall and Ferrovial were previously part of the Fusion consortium with Morgan Sindall, which unsuccessfully bid for the northern and southern sections of HS2 phase one.
HS2 avoided going out to public tender again, because EU procurement rules allow for replacement to be appointed in the event of insolvency.
Chiefs at HS2 also argued that re-procurement would also be unfair to surviving partners Kier and Eiffage.