Reproducing the continuous compositional and cellular transitions found in native tissues remains a major challenge in extrusion-based bioprinting, which typically generates constructs composed of discrete regions and imposes stringent rheological requirements on bioinks. Microfluidic bioprinting offers new opportunities to reproduce tissue heterogeneity by enabling controlled mixing of biomaterials and cell populations during extrusion. In this study, we present a coaxial microfluidic bioprinting strategy for fabricating hydrogel scaffolds with continuous cellular gradients using low-viscosity self-assembling peptide bioinks. The system combines three independently controlled syringe pumps with a 3D-printed coaxial nozzle containing a screw-like passive mixer. Two low-viscosity RADA16-I peptide solutions containing different cell populations are mixed in situ, while a methylcellulose–alginate shell stabilizes the filament during printing and supports post-printing self-assembly of the core hydrogel. Computational simulations confirm efficient mixing within the nozzle, and fluorescence imaging demonstrates smooth compositional transitions along printed filaments. The system enables the fabrication of scaffolds containing co-culture gradients of endothelial cells and mesenchymal stem cells, which remain viable and display cell-type-specific organization. Overall, this approach enables the printing of soft peptide hydrogels and the fabrication of biomimetic constructs with continuous cellular transitions, highlighting its potential for tissue engineering and regenerative medicine.