The attainability of fabricating ground-breaking boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of boron carbide + silicon powder mixtures at only 1400 °C was examined. To start with, it is exhibited that boron carbide can be fully densified at 1400 °C if ~20 vol% silicon aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and silicon carbide (minor phase). The formation of these composites is due to the fact that silicon acts as a reactive sintering additive during SPS. Lower and higher proportions of silicon aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free silicon. Importantly, it is also shown that these ground-breaking boron carbide–silicon carbide composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Also, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient silicon aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of silicon carbide. And finally, it is demonstrated that during SPS the original boron carbide goes through a gradual isostructural crystallographic transition towards a silicon-doped carbon-deficient boron carbide that is more relevant with increasing proportion of silicon aids, and it is determined that the carbon source for the formation of silicon carbide is almost exclusively the carbon exsoluted from the boron carbide crystals themselves during their isostructural transition.