In this study, the comparison between the orbit electron density and total ion density measured by payloads of Tri-GNSS Radio-Occultation System (TGRS) and Ion Velocity Meter (IVM) onboard the FORMOSAT-7/COMSIC2 (F7/C2) satellites is presented. The collocated TGRS and IVM observations for each of the F7/C2 satellites at ~715 km and ~540 km are evaluated during the whole year of 2020. Comparative analysis reveals that the TGRS and IVM density observations have high correlation coefficients of 0.92-0.96 for each of the F7/C2 satellites. The mean differences are around -0.03´104-0.02´104 cm-3 with standard deviations ranging from ~0.91´104-2.18´104 cm-3, demonstrating a good agreement between the independent TGRS and IVM observations. Furthermore, the collocated observations are utilized to examine the global spatial and temporal variations of the topside ionosphere. The results show that the morphologies of the topside ionosphere in TGRS orbit electron density are nearly identical to the IVM ion density, suggesting that both F7/C2 payloads reliably produce accurate topside ionosphere observations. We found that the TGRS orbit electron density tends to be 10-30% smaller than the IVM ion density in the daytime equatorial ionosphere region at both high and low orbits, but the density differences become smaller of approximately -30-0% during the nighttime. These density differences could be due to the error caused by the spherical symmetry assumption in TGRS orbit electron density estimation. Observing system simulation experiments are further performed to evaluate the relative errors between the TGRS and IVM density observations at high and low orbits.