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One of the most exciting results in modern cosmology has been the discovery of the accelerated expansion of the Universe. It could be due to an V1.0 energy component (i.e. comma dark energy) or the modification of general relativity. In order to illuminate the V1.0 nature of the observed cosmic acceleration comma ESA.s Euclid and NASA.s Roman Space Telescopes will be launched in 2022 and virgul2026 respectively. They will complement each other in probing cosmic acceleration with high precision and accuracy comma and need tight requirements on spectrophotometry to unprecedented accuracy. Accurate absolute spectrophotometry is vital to determine the fraction of baryonic matter turned into stars comma for galaxy and supernovae surveys comma and to enable legacy science. Extreme care must be taken to control systematic errors and biases. We propose to establish six hot White Dwarfs as stable comma spectrophotometric IR standards for use by Euclid and Roman. The targets are suitable for spectrophotometry with the WFC3-IR grisms comma and chosen to lie near both ecliptic poles within the Euclid and Roman CVZ. This allows for year-round accessibility. The stars lie within two planned comma frequently repeated comma Euclid deep fields comma and in a likely location to be observed by Roman for calibration purposes. Although not formally an HST calibration proposal comma our 11-orbit program will be vital in tying Euclid-Roman deep field spectra of faint galaxies to well-calibrated HST standards. It will also cross-calibrate the spectrophometry to broad-band IR colors of these faint high ecliptic-latitude sources. This proposed program is of critical importance to the successful calibration of both Euclid and Roman.