Catalyzes both the synthesis and degradation of fructose 2,6- bisphosphate. {ECO:0000269|PubMed:10077634, ECO:0000269|PubMed:17499765, ECO:0000305|PubMed:16316985}. This is the function of PFKFB3 (6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3, Ensembl gene identifier ENSG00000170525).