Typical local (z ≈ 0) BH-BH mergers formation scenarios for model M480.B, together with the alternative evolution of the same system in models M380.B and M481.B. For all models, we begin with the same initial conditions, massive system of 145 M_⊙ primary and 83 M_⊙ secondary on the orbit of 263 R_⊙ and with metallicity Z = 0.0001. In all scenarios, the primary initiates TTMT when it leaves main sequence and begins to expand in thermal timescale during the HG phase. When TTMT finishes, in case of models M380.B and M480.B secondary looses envelope. A BH with stripped helium core companion on the orbit of 350 R_⊙ is left. In model M481.B due to modified condition for switch between the TTMT and nuclear-timescale stable MT, and the “safety” condition (read Sect. 3.2) when TTMT ends, the system goes through the CE phase. As the donor is a HG star, the CE ends with the system merger leaving behind an unevolved single star. After 3.3 Myr, in models M380.B and M340.B, the primary finishes evolution and collapse to a 20 M_⊙ BH remnant. Next, after the secondary leaves main sequence, due to more restricted conditions for CE development, in cases of model M480.B system goes through TTMT instead of CE phase as in standard M380.B model. At the onset of TTMT, the secondary (donor) is over four times more massive than a BH companion. During the TTMT, the donor star looses ∼60 % of its mass together with the system orbital angular momentum. This leads to the significant orbit tightening by a factor of ∼12. Efficient orbital angular momentum loss during TTMT allows for the subsequent formation of 20 M_⊙ BH and 30 M_⊙ BH systems to merge in Hubble time. In the scenario for M380.B, the highly unequal mass system enters the CE phase with the HG donor. The binary system does not survive CE phase leaving behind a single BH. The “He” in the diagram stands for stripped helium core.