The lack of response may be related to your question about “dual master cylinders” which are seldom used on anything except race cars where the ability to change the front / rear brake bias during a race is important.
I suspect that you are actually interested in a tandem master cylinder which splits the front and rear hydraulic circuits and improves safety by eliminating the single point failure mode. If so, the master cylinder from a mid-’70’s Datsun (Nissan) PL620 pickup truck has a 3/4″ bore and will bolt up to the Alpine / Tiger mounting points. You will have to deal with the metric line fittings with the appropriate adapters. The tandem M/C is longer than the Alpine M/C and may result in intake manifold / carburetor clearance issues. I am not far enough along to comment on the performance.