  Line intensity mapping (LIM) serves as a potent probe in astrophysics,
relying on the statistical analysis of integrated spectral line emissions
originating from distant star-forming galaxies. While LIM observations hold the
promise of achieving a broad spectrum of scientific objectives, a significant
hurdle for future experiments lies in distinguishing the targeted spectral line
emitted at a specific redshift from undesired line emissions originating at
different redshifts. The presence of these interloping lines poses a challenge
to the accuracy of cosmological analyses. In this study, we introduce a novel
approach to quantify line-line cross-correlations (LIM-LLX), enabling us to
investigate the true signal amidst instrumental noise and interloping
emissions. For example, at a redshift of approximately $z\sim3.7$, we observed
that the measured auto-power spectrum of [CII] exhibited substantial bias, from
interloping line emission. However, cross-correlating [CII] with CO(6-5) lines
using a FYST-like experiment yielded a promising result, with a Signal-to-noise
ratio (SNR) of $\sim 10$. This measurement is notably unbiased. Additionally,
we explore the extensive capabilities of cross-correlation by leveraging
various CO transitions to probe the tomographic Universe at lower redshifts
through LIM-LLX. We further demonstrate that incorporating low-frequency
channels, such as 90 GHz and 150 GHz, into FYST's EoR-Spec-like experiment can
maximize the potential for cross-correlation studies, effectively reducing the
bias introduced by instrumental noise and interlopers.
