  We report the transport, thermodynamic, $\mu$SR and neutron study of the
Ca$_{0.74(1)}$La$_{0.26(1)}$(Fe$_{1-x}$Co$_{x}$)As$_{2}$ single crystals,
mapping out the temperature-doping level phase diagram. Upon Co substitution on
the Fe site, the structural/magnetic phase transitions in this 112 compound are
suppressed and superconductivity up to 20 K occurs. Our measurements of the
superconducting and magnetic volume fractions show that these two phases
coexist microscopically in the underdoped region, in contrast to the related
10-3-8 Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_x$)$_{2}$As$_{2}$)$_{5}$
compound, where coexistence is absent. Supported by model calculations, we
discuss the differences in the phase diagrams of the 112 and 10-3-8 compounds
in terms of the FeAs interlayer coupling, whose strength is affected by the
character of the spacer layer, which is metallic in the 112 and insulating in
the 10-3-8.
