We present a novel strategy involving the lanthanide dopant-induced formation of ultrasmall (sub-10 nm) uniform Ln³⁺:BaF₂/Ln³⁺:SrF₂ active-core/active-shell architectures. The lanthanide ions doped in the shell are demonstrated to play a key role to retard the growth of the core/shell nanocrystals. Particularly, adopting ∼3 nm Tm³⁺,Yb³⁺:BaF₂ nanocrystals as cores prepared by a solvothermal reaction, growth of Gd³⁺,Nd³⁺:SrF₂ shells is successfully induced on the surfaces of these cores through a thermal decomposition process, forming ∼7 nm highly uniform and monodisperse Tm³⁺,Yb³⁺:BaF₂/Gd³⁺,Nd³⁺:SrF₂ active-core/active-shell nanocubes. In this architecture, the Gd³⁺,Nd³⁺:SrF₂ shell not only benefits the enhancement of the near-infrared to near-infrared upconversion luminescence of the Tm³⁺,Yb³⁺:BaF₂ core, but also acts as the host to realize the near-infrared to near-infrared downconversion luminescence of Nd³⁺ dopants and the paramagnetism of Gd³⁺ ones. Importantly, the doping of Tm³⁺/Yb³⁺ and Nd³⁺ into the core and shell respectively effectively suppresses the adverse energy transfer from Tm³⁺ to Nd³⁺ as well as from Nd³⁺ to Yb³⁺, resulting in lessening of the quenching for both Tm³⁺ upconversion and Nd³⁺,Yb³⁺ downconversion emissions. These hydrophobic core/shell nanocrystals are further converted into hydrophilic ones using thioglycolic acid as the surface ligand. The sub-10 nm water-soluble active-core/active-shell architectures with near-infrared to near-infrared dual-modal luminescence and proper paramagnetism may find potential applications in biomedical imaging and detection.