How should you name the solid-solution compound ScY(SeO4)3?
This is similar to a double salt, but because of the chemical similarities between scandium and yttrium, $\ce{ScY(SeO4)3}$ is a solid solution in this case. And the answers is you just name the cations in alphabetical order, then the anions like any normal salt. Thus the name is scandium yttrium selenate.

If the ratio between cations had a large difference such as $\ce{Sc_{1.9}Y_{0.1}(SeO4)3}$ It might be called yttrium-doped scandium selenate, but could still correctly be called just scandium yttrium selenate too.