If my memory serves, the universal triggers (UT) design was first suggested by Horatio Philips, the notorious gun editor of The Field in the last decades of XIX century (he also invented the Vena Contracta. What a guy!).
The list of makers that tried it includes two Russian makers, TsKIB SOO and IzhMech (Baikal). TsKIB made a few of their MTs-11 (Beesley patent self-opener) in early 1960s, and Baikal tried it twice, with Izh-59 over/under also in 1950s and with Izh-43 (aka Stevens 411 aka Remington Spartan 2xx) in mid-1980s. In all instances after a short trial the idea was abandoned?
At the first glance UT like a winner: all advantages of ST and DT in one gun! Well, not quite. You don't get the biggest advantage of ST: uniformity of handling. UT still has two triggers, and requires adapting the grab of the rear hand for the rear trigger.
What's worse, UT also combines all disadvantages of ST and DT. From the ST, it inherits greater sophistication and, consequently, lower reliability. From DT - a chance of confusion between the triggers (if you've never pulled the trigger of the wrong barrel, you've never shot a double with DT).
But the real problem with the UT is that the disadvantages of ST and UT are multiplied. Two single triggers are statistically twice as likely to fail as one ST. And while with DT you have two possible sequences of trigger operation, with UT you have four! Terminator could've handled that, but for a human mind in the Zeitnot of shooting flying it's too much.
