Well, shit. Adapsin says that it is "in addition" to grade reductions, and also says to "round normally" (WTF? Since when do you round essence costs at all?). Second-hand multiplies. Biocompatibility doesn't say. I think Biocompatibility working like Adapsin makes more sense, as Adapsin is also a cost reduction.
On further thought, Way+Gaesa is also additive. I can't think of any examples of two discounts (or two markups) that are explicitly multiplicative; only Second-Hand Ware (which is a discount and a markup multiplying).
Based on this, I'm inclined to stick with additive (ie, both of them discount the base value), since that seems to be the standard for how SR does it. I'm going to take another look through the rules and possibly a straw poll, though.