Do individual crystals resulting from evaporating a mixture of two ionic compounds contain both cations/anions within the crystal lattice?
Crystals with randomly intermixed ions only occur when the ions have very similar diameter (really the same spatial charge distribution in its exterior, but alkali ions are in very good approximation spherical) and same charge. That would be called a single phase solid solution.

However, $\ce{Na+}$ and $\ce{K+}$ have quite different diameters ($r=102$ resp. $\pu{138 pm}$). Your mixed solution will crystallise as NaCl with a low amount of potassium ions and KCl with some potassium replaced by sodium ions. In other words, your isotropic solution decomposes into a solid two-phase mixture.

The two substances will surely have a different tendency (also temperature dependent) to include such defect ions, and the two compounds have different solubility, so one will precipitate first, so the concentrations in the residual solution changes greatly during the process, and consequently also the defect concentration in the single crystals.