Earth Observing Systems (EOS) are a valuable tool for vegetation monitoring. Coupling information from different spaceborne sensors allows reducing the revisit time and a more accurate Earth surface processes monitoring. However, combining surface reflectance (SR) from different EOS requires compensating for atmospheric disturbances and viewing angle induced effects. The objectives of this study were i) to compare the atmospherically corrected Sentinel-2 (S2) and WorldView-3 (WV3) SR with ground-truth spectra acquired with a FieldSpec and ii) to propose and validate a signal normalization procedure for coupling S2 and WV3 imagery. The S2 imagery showed a reliable match with field spectra, however SR from WV3 was significantly different. These discrepancies likely resulted from the off-nadir acquisition angles. The signal normalization procedure, based on calibrating each WV3 band against ground-truth spectra, allowed an accurate coupling of S2 and WV3 imagery ($\text{RMSE} < 0.007$ in visible bands and $\text{RMSE}=0.018$ in the NIR band).