If you like snow globes, that's the way to do it! Alk and calc in a small volume will cause for precipitation of calcium carbonate......snow! When dosing two part, dose into your sump or DT, and not at the same time. Wait for one to dissipate before adding the other. Dependent on flow, this might just be a minute or two.
No you cannot dose 2 part in your ato. You cannot mix ca. and alk. together. You have to use a dosing pump or manually dose each element seperately. You need to give some time between dosing the ca. and the alk.