The investigation of the tin-rich part of the ternary system Co/Ni/Sn yielded the phase Co1-xNixSn2 with the range of composition 0.23(3) < x < 0.59(3). When using a large excess of tin, Co1-xNixSn2 crystallizes at 500 degrees C in a structure isotypic to that of tetragonal PdSn2 whereas a modification with the orthorhombic CoGe2 structure type always forms from a stoichiometric mixture of the elemental components or from a tin melt at temperatures above 550 degrees C, The structures of Co0.625Ni0.375Sn2 were determined by single-crystal X-ray diffraction methods (PdSn2-type space group I4(1)/acd, a=6.2360(5) Angstrom, c=23.588(2) Angstrom, Z=16; CoGe2-type space group Aba2, a=6.2439(4) Angstrom, b=62493(4) Angstrom, c=11.778(1) Angstrom, Z=8). Both structures have the same building unit consisting of three consecutive planar-nets 4(4), 3(2)434, 4(4) formed by tin atoms. The condensation of the building blocks in the c direction gives rise to different stacking sequences. In the PdSn2-type, a ABCD sequence is realized and the c axis is doubled compared to the ideal CoGe2-type with AB stacking. Electron diffraction and high-resolution electron microscopy studies revealed the existence of other, complex, stacking variants as well as stacking faults in the Co1-xNixSn2 system.