Under the trends of multifunctionality, tunability, and compactness in modern wave-based signal processors, in this paper, we propose a polarization-multiplexed graphene-based metasurface to realize distinct mathematical operators on the parallel time-domain channels enabled by vertical and horizontal polarizations. The designed metasurface is composed of two perpendicularly-oriented graphene strips for each of which the chemical potential can be dynamically tuned through a DC biasing circuit. The programmable metasurface exhibits two orthogonal channels through which the time-domain input signals are elaborately processed by separate mathematical functions. Several illustrative examples are presented demonstrating that the proposed device can operate on different time-domain analog computing modes such as fractional-order differentiator and phaser at the same time. The strategy introduced in this paper will enable real-time parallel temporal analog computing and has potentially essential applications in terahertz spectroscopy architectures, communication systems, and computing technologies.