This work explores the synergies between N-rich (Chlorella pyrenoidosa) microalgae and N-deficient (Undaria pinnatifida) macroalgae for the production of N-containing hydrochar and solid biofuels via co-hydrothermal carbonization (co-HTC). The impact of the feedstock (each alga alone and all possible binary mixtures) was comprehensively assessed under different temperatures (180-260 °C) and times (60-240 min). The synergies between micro and macroalgae governed product distribution, nitrogen transformation pathways, and hydrochar quality, with these effects varying by processing conditions. Biomass synergies enhanced hydrochar quality at lower temperatures through deoxygenation reactions and/or liquid-phase repolymerization. In contrast, at higher temperatures, interactions between carbohydrates and proteins via solid-phase Maillard and Mannich reactions decreased hydrochar fuel quality but enriched nitrogen functionalities, such as pyridine-N. Optimization revealed that high N retention and hydrochar yield (up to 23%) were achieved by mixing up to 50 wt% macroalgae with microalgae at 223 °C for 174 min, maintaining functional N content (6 wt% N, 16% pyridine-N). Additionally, an energy-dense hydrochar (34% yield and 26 MJ/kg HHV) was synergistically produced by co-treating 70 wt% microalgae and 30 wt% macroalgae at 180 °C for 60 min. This synergistic algal approach highlights the potential of synergistic algal co-HTC to enhance nitrogen circularity, improve feedstock flexibility, and support sustainable biofuel and material production from marine resources.