Busan 77 is the trade name for poly-yadda-yadda-dichloride, one of many names for the same thing, just much easier to type.
I agree with [Plantbrain], Algaefix alone has no useful effect on BBA/staghorn, at least with recommended dosage and method.
Some of the Metricides contain an undisclosed surfactant, and people generally avoid them for that reason. Ironically, the surfactant is likely Busan 77, or something very much like it. Such combos are well-documented in medical and scientific areas, the surfactant improves cell absorption of glutaraldehyde, and therefore makes the combo a more effective sterilizer.
So in principle at least, there is a synergy between Algaefix and Excel.
Though I've been using Metricide 28 for about a year, simply because I ordered it by mistake. The amount of surfactant is small, and the dosage even smaller, so the additional ingredient is too dilute to have any noticeable effect.
Dosing Metricide and Algaefix together, I may have noticed a small synergy. But it's too small to say for sure. Nothing like the obvious synergy between H2O2 and Excel.