To be honest…
The second IC is not needed until Japan hits their build limit with the first 2 IC’s… 8 units Japan, 3 FIC.
With an IC in J1 (though I prefer J2 and try to get that IC in India), and tranny builds for Japan early, Japan can drop 4 INF, 4 ARM via TRN anywhere along the Pacific rim… SFE, Bury, Manch, Kwang, FIC… and still return to fill up again and drop the same 4x4 the next turn. Add in 3 ARM from FIC, and you are talking 42 IPC’s worth of units.
It takes Japan a while to build up to that level of income (substituting ART for ARM from Japan while income is in the high 30’s) so that they need a 3rd IC.
Even then, mixing up the build… saving IPC’s to add a FIG or a BOM every few rounds, may be a more optimum way to go, rather than dumping the IC’s into a second mainland IC.
Of course, for those who prefer to mass INF, then yes, 2nd and even 3rd IC’s is a good idea. But, even though I failed to adequately counter that move in two previous games (credit to Octo), I still feel it is sub-optimal to a more balanced offensive punch of INF w ART/ARM support sent in via TRN from Japan.