FYI Phil, no dual alternator belts in our RWDs since the 140s and early 240s. In part, dual belts were probably used to help deal with the added mass of turning the viscous fans. The early viscous fans were a bit heavier than the later ones. Any dual pulleys you see now are vestigial or done in order to accommodate a number of fitments with the same alt. Volvo and others like Goodyear sold belts in matched pairs to assure equal characteristics. You paid extra for the privilege. As matched pairs weren't always commonly sold, the next best option was to try finding belts with the same lot number. A number of such comments here on the Brickboard in its early days. Way back, when you ordered the two belts from FCP (before they became FCPEuro) they advertised they would pick matching lot numbers out of the parts bin. Dual belts were problematic in that over time the outer belt stretched a bit more than the inner belt, so one would start to squeal before the other. The advice was rather than tensioning to make the squeal go away and as that would place additional load on the water pump and alt bearings, to switch the belts back to front at every oil change, a habit I made at the time. One minor issue was that when one of the dual belts let go, it would occasionally get caught in the other belt and take it out.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now