In Arabidopsis, microRNA-directed cleavage can define one end of RNAs that then generate phased siRNAs. However, most miRNA-targeted RNAs do not spawn siRNAs, suggesting the existence of additional determinants within those that do. We find that in moss, phased siRNAs arise from regions flanked by dual miR390 cleavage sites. AtTAS3, an siRNA locus important for development and conserved among higher plants, also has dual miR390 complementary sites. Both sites bind miR390 in vitro and are functionally required in Arabidopsis, but cleavage is undetectable at the 5' site--demonstrating that noncleavable sites can be functional in plants. Phased siRNAs also emanate from the bounded regions of every Arabidopsis gene with two known microRNA/siRNA complementary sites, but only rarely from genes with single sites. Therefore, two "hits,"--often, but not always, two cleavage events--constitute a conserved trigger for siRNA biogenesis, a finding with implications for recognition and silencing of aberrant RNA.