The aim of this work was to explore the possibility of mixed feedstocks utilization using acid-catalyzed steam pretreatment in the context of forest biomass. Three steam pretreatment conditions were applied to characterize the interactions between spruce (softwood) and poplar (hardwood) that were concurrently processed by SO2-catalyzed steam pretreatment and enzymatic hydrolysis. The effects of blending feedstocks were evaluated in terms of composition and enzymatic digestibility. The combined sugar yields after steam pretreatment and enzymatic hydrolysis ranged from 58% to 71%. No synergistic or antagonistic interactions were observed in the concurrent use of spruce and poplar—our linear interpolation model accurately predicted the overall sugar recovery after steam pretreatment and enzymatic hydrolysis for a 50:50 blend to within 3%, based on the results of the individual feedstocks.