Not that the two-box arrangement was a necessity but the RS-9900 was one of the very last overly over-sized beast the 70s ever produced.
It was an all-out effort to make it the best, the biggest, the most impressive, the most... Most !
By 1977, however, down-sizing was already just around the corner (just like the cost-cutters) and the RS-9900 didn't sell that much and is today, unlike the big Naks or Pioneers, a true rarity.
Because the two-box arrangement really wasn't a necessity - market-wise.
As often with late 1970s japanese monsters, we westerners remember them better and, like a few others, the RS-9900 was mainly an export item ; if quite rare for us, it remains a very obscure and barely mentioned über-rarity in Japan.
Even Technics in the Stereo Sound "bible" published in 1978 remained very discrete about it, spending more time talking about the RS-M50 or RS-M85.
Or the RS-1500U series :)
It is however possible Technics kept so discrete about this monster because of the then ongoing development of DD4 : a four direct drive cassette mechanism which finally remained unproduced.
It was to be the most most most, again, but with one BSL direct-drive for each reel drive and each capstan - 2 + 2 = 4.
However, when DD4 would've been ready, 1978, cost-cutters were in and... cost-cutting.
Available in Japan under the RS-690U name.
You can see large "nudies" or both units here here (scroll down the page).