This study provides a novel method for the fabrication and microstructural manipulation of high-performance dual-layer hollow fiber membranes for H2/CO2 separation at elevated temperatures. For the first time, hollow fibers consisting of an outer-selective layer made from polybenzimidazole (PBI) and sulfonated polyphenylsulfone (sPPSU) blends and an inner-support layer made of polysulfone were prepared. sPPSU was chosen because it performed as an ionic-crosslinker towards PBI and it had a higher permeability than PBI. The as-spun hollow fibers were chemically cross-linked by α,α′-dibromo-p-xylene (DBX) and then heat treated to enhance their gas separation performance for H2/CO2 separation. Experiments revealed that sPPSU interacted with PBI at the molecular level and increased the interchain space within PBI, leading to an increment in H2 permeance. The covalent crosslinking reaction between PBI and DBX resulted in the outer-selective sPPSU/PBI layer with a smaller free volume but a greater H2/CO2 selectivity. The dual-layer hollow fiber membrane cross-linked by 3% DBX and annealed at 120 °C (i.e., HSP-10-3%DBX-120) has a H2 permeance of 16.7 GPU and an H2/CO2 selectivity of 9.7 at 90 °C and 14 atm. The fabrication of dual-layer hollow fibers may open up a new approach to produce useful membranes for hydrogen purification and CO2 capture at elevated temperatures.