TTV-TDV diagrams of a Jupiter-like planet with 2 moons. Top panels assume a 2:1 MMR akin to the Io-Europa resonance; bottom panels assume a 4:1 MMR akin to the Io-Ganymede resonance. Models represented by black solid lines assume that both moons are as massive as Ganymede. In the left panels, blue dotted lines assume the inner moon is twice as massive (M_s1 = 2 M_s2 = 2 M_Gan), and red dashed lines assume the inner moon is thrice as massive. In the right panels, blue assumes the outer moon is twice as massive (M_s2 = 2 M_s1) and red assumes the outer moon is thrice as massive. The inner loop becomes invisibly small for low M_s1/M_s2 ratios in the top right panel.