Tandem warheads were introduced in the last round of Cold War developments, i.e. usually developed in the mid- to late 80s and deployed in the early to mid 90s. Usually in heavier systems first, first systems in the West to introduce them were TOW-2A in 1987 and Eryx, developed in the 1983-85 (and deployed in 1993). The SU introduced the PG-7VR in 1988, and 9M113M around 1992-93, Western Europe got the Milan-2T around the same time (though i think most went straight to Milan 3 three years later).
For the most part, tandem warheads - other than perhaps PG-7VR, Eryx and TOW-2A - weren't really widespread until the late 90s, when other systems were upgraded. And even those three would have been "rare" at the time.