The stepped quenching would no doubt increase the resistance to SCC of Aluminum alloys but would also lower the mechanical properties like strength and hardness. In case of 7xxx series the decrease in strength is about 10-15% which is acceptable considering susceptibility to SCC. I think in case of 2xxx series the decrease in mechanical property would be higher.

In case of 7xxx series the precipitates formed are MgZn2 which are more active (less nobler) than Al Matrix. Therefore the mechanism of intergranular corrosion (leading to crack Propagation in SCC) is due to Dissolution of MgZn2 particles along grain boundary (Galvanic couple is formed between Al Matrix and MgZn2 particles along GB). By stepped quenching we obtain an discontinuous precipitates at grain boundries thus reducing the Dissolution of MgZn2 particles. While in normal T6 quenching we obtain continuous network of MgZn2 particles which are more susceptible to IG corrosion and in turn crack Propagation during SCC.

While in 2xxx series the precipitates along grain boundries are Al-Cu-Mg intermetallics which are more nobler than Al Matrix. Thus during IG corrosion the galvanic couple is formed like 7xxx series but here Al matrix around these particles dissolute or corrode . It does not matter if these precipitaes are continuous or discontinuous unlike 7xxx series.Thus I believe  stepped quenching would not  help in this case like 7xxx.

Thank you Abhishek Ramakrishna Shetty. That helped.

I have now come across step homogenising that is done before solution treatment. Can you enlighten on this aspect too?

It is important to dissolve non-equilibrium eutectics, eliminate segregation and precipitate dispersed and small coherent (Al3Zr) phase particles in the matrix to have higher strengths and resistance to SCC 

The homoginising is done to have finely dispersed dispersoids formed because of either Cr/Mn/ Zr elements. These form either coherent or inchoherent precipitates. Having finely dispersed disperoids the fracture toughness can be increased. 

Stepped homoginising is done usually in 2 steps. One at temperature less than 400 deg C and 2 nd at temperature higher than 400. The temperature depends on various parameters like alloying elements and composition. By this we can obtain very fine and distributed dispersoids (more fine and dispersed than one step). The second step other this also would dissolve course MgZn2 / Al2CuMg constituents, but these might re-precipitate later.

These dispersoids are recrystallization inhibitors. These particles which ranged in size from 20 to 35 nm, helped stabilize the fine-grained microstructure are attributed to the pinning of gain boundaries by fine dispersoid particles. However, a high coherency mismatch is observed, resulting  in significant lattice strain, which serves to block dislocation motion and to prevent grain grow lead to the increase in strength.

The dispersoid particles suppress the planar slip of dislocation and the smaller grain size restricts the density of dislocation at the grain boundary. Therefore, the concentrations of stress at the grain boundary may be reduced significantly due to the grain refinement effect of Al3(Sc,Zr), which decreased the susceptibility to corrosion.(Reference: Effects of Microstructure on the Mechanical Properties and Stress Corrosion Cracking of an Al-Zn-Mg-Sc-Zr Alloy by Various Temper Treatments- Ling-Mei Wu)

Generally this step would both increase strength and decrease the susceptibility to corrosion. However the actual reason/mechanism for this is not exactly known.