Dual Systems was founded in the late 1970's, during the heyday of the S-100 bus (IEEE-696; 24 address lines, and 16 data lines, can do either 8 or 16 bit transfers, maximum bus frequency 5MHz) computers. It was originally founded to produce a variety of peripheral boards (A/D and D/A converters, serial interface boards, and so on), but once they had designed a CPU board around the Motorola 68000 CPU, they decided to go into the systems business.
After a brief flirtation with CP/M-68K, Dual decided that their system would run UNIX. In order to get UNIX to run on the 68000, first a C compiler had to be found, a UNIX source license would have to be gotten from the Western Electric division of AT&T, and then a "porting" effort would have to be done. Dual approached Jeff Schriebman to do the port, and helped him set up a company whose business would be porting UNIX to a variety of different systems. That company was UniSoft Systems ("Berkeley's Port Authority").
Dual was UniSoft's first customer, but UniSoft eventually supplied the initial "port" of UNIX to almost every Motorola 68000-based system during the 1980's, including Sun Microsystems, Silicon Graphics, Apple Computer, Heurikon, CoData, Momentum, Pixel, Plexus, and Motorola itself.
Dual was unique in that it had used the S-100 bus as the basic backplane of its original systems, when almost everyone else who followed used Intel's MultiBus. Dual bought S-100 boards from CompuPro (formerly Godbout Electronics), Morrow Designs (formerly ThinkerToys), Macrotech, and some of the other well known S-100 design houses of that time. Dual also designed its own SMD disk controller, four port RS-232 serial controller, Pertec interface 9-track tape controller, DRAM and SRAM memory boards.
When Motorola announced the 68020, Dual decided to switch system busses from S-100 to VME (a Motorola bus design; 32 address and data lines, can do 8, 16, or 32 bit transfers), and to design a multiprocessor UNIX system. The VME bus is faster than the S-100, and is better suited to multiprocessor and multiple bus-master systems than the Intel MultiBus (or MultiBus II). It was also clear that the S-100 bus was reaching the end of its design life. Dual successfully designed a new CPU board around the 68020 with 512K of DRAM right on the board (VME boards are 20% bigger than S-100 boards, and do not require power regulation right on the board since bus power is regulated system-wide).
Dual did not keep up with the market, however; they never produced a system with a graphics console - just a computing box for use with terminals. They might have held on longer, had they adopted the model that MIPS Computer used prior to their acquisition by Silicon Graphics: MIPS simply added X Window System terminals (from the likes of Network Computing Devices) to their system configuration.
In the end, Dual was acquired in the late 1980's by Northstar (another S-100 systems vendor trying to remake itself), and their technology was sold off to whatever interested parties could be found (I saw Dual's VME 9-track tape controller being sold by Emulex at a trade show, long ago).