Cost is the reason in most cases.. If you can get two single 300ZX turbo's then yes but fabrication is a little more involved..
The reason Supra's etc hp single is again due to cost.. But turbo tech has come along way the last few years..
It doesn't matter either way, but its much easier on an inline six to make a single turbo system..
The ZX came in single and twin turbo configuration.. The early ZX's had single and domestic Japan [Same engine in bigger car] run single gigger T3 turbo..
Into you can pick up a T70 Master power for under $1k new.. I doubt you'd get second hand twins for too much under that.. Plus it costs heaps to overhaul twins.. The have to be high flowed so both have to be overhauled...