The spectral coexistence between low Earth orbit (LEO) and geostationary orbit (GEO) satellite systems has emerged as a promising strategy due to the spectrum scarcity and demand for high throughput. A main issue for this coexistence is the management of in-line interference, caused by LEO satellites to the GEO users (GUs) when LEO satellites move near a line-of-sight path between the GEO satellite and the GUs. Rate-splitting multiple access (RSMA) has proven to be robust for interference management, and this paper put forth a novel RSMA with a super-common message for LEO-GEO coexisting satellite systems (CSS). By employing a super-common message that GUs can decode, GUs can mitigate the in-line interference through successive interference cancellation. Moreover, we aim to achieve high demand satisfaction, being aware of the imbalanced traffic demands among users. To this end, the precoder is designed to consider both the interference leakage from LEO satellite to GUs and the target traffic demands. Numerical results validate the effectiveness of our proposed framework, ensuring seamless connectivity even in the GEO-LEO in-line interference regime, particularly under imperfect channel state information at both the transmitter and receiver.