tuo: modifying both nouns, though agreeing with the second.
 quidquid amas Catullum: i.e. in proportion to the love you bear Catullus: a variation on the colloquial phrase si me amas in exhortations; cf. Pl. Trin. 244 “da mihi hoc, mel meum, si me amas, si audes” ; Ter. Heaut. 1031 cave posthac, si me amas, unquam istuc verbum ex te audiam; Cic. Att. 5.17.5 si quicquam me amas, hunc locum muni.
 si placet Dionae: a variation on the phrase si dis placet, sometimes used in the sense of dis iuvantibus of completed actions; cf. Pl. Capt. 454 “expedivi ex servitute filium, si dis placet” . Dione is mentioned in Hom. Il. 5.370 as the mother of Aphrodite, but Catullus apparently has in mind Venus herself; cf. Bion 1.93; Theocr. 7.116; Pl. Mil. 1414; and the Augustan and later poets often, as Verg. Ecl. 9.47 “ecce Dionaei processit Caesaris astrum” ; Hor. Carm. 2.1.39 “Dionaeo sub antro” .