Age-metallicity relation (AMR) for the twin GCs. The red solid curve represents the AMR including the twin GCs, with a formation time of t_f = 13.5 Gyr, effective yields ρ = 0.077Z_⊙, and [α/Fe]=+0.20. The black solid and dashed curves represent the AMR respectively for in situ and ex situ bulge metal-poor GCs. The open circles indicate the data from Kruijssen et al. (2019), while the blue triangles show the old moderately metal-poor clusters already presented in Souza et al. (2024) for the metal-poor bulge GCs. The green and purple stars mark the positions of NGC 6553 and NGC 6528, respectively, and highlight their consistency with the AMR trend.