Well, if the japanese actually keep to the treaty and build 35,000 tons BBs, they could get the first pair ready in 1940 and second in 1943. Though i would think they could surely fit more than 9x 36cm guns on 35,000 tons, KGV had 10 while the US were contemplating North Carolina with 11 or 12 such guns.
Anyway, having them earlier is one of the attraction of smaller 45,000 tons or so Yamatos. The second pair of hulls could be launched in 1942, and if the events are about same as in OTL (read, Midway etc.) then they'd have both available to transform into armoured deck CVs like Shinano, a bit smaller but still formidable, maybe even in time of Marianas. They probably could carry about 65-70 modern (A7M, B7A) planes, or over 80 older ones (A6M, B6N, D4Y).
OTL Shinano could carry 80 and 90-95 respectively, i know what is written in literature and what is believed about her concept and aircraft capacity, but recently i've read info that the IJN was intending to use it as a normal CV afterall (yes, the initial concept was for a heavily protected forward floating base shuttling planes from other carriers and carrying large amount of spares etc.etc., but they changed their minds as the war progressed) those being the number of aircraft she would actually be able to operate as a normal CV.