The PROSPECT and STEREO reactor antineutrino experiments have made world-leading measurements of the 235U antineutrino energy spectra from highly-enriched uranium (HEU) research reactors using liquid scintillator based detectors. At HEU reactors, 99% of the antineutrino flux comes from 235U, providing a direct measure of the energy spectrum and antineutrino flux from a single isotope. STEREO and PROSPECT have provided independent measurements with different systematics from detectors at ILL (France) and HFIR (US). The work with STEREO directly combines the two pure 235U measurements and jointly unfolds the spectra to provide an improved result in antineutrino energy, independent of assumptions regarding other isotopes' contributions. In this poster, I will present the results of this analysis.