ppm might in principle be mass-ppm (micro-g per g) or atom-ppm (1 atom in a million). However, in geochemistry ppm typically mean mass-ppm. So when you divide these ppm values one by another, you will get the mass ratio of K/Rb. If you are rather interested in the atomic ratio [K]/[Rb] (or atomic concentration ratio), you have to correct this for the atomic masses: 

[K]/[Rb] = mass-ppm(K) / mass-ppm(Rb) * 85.5/39.1

Hi. It depends on the outcome(s) desired. If it is simply a ratio then dividing the concentration of the one by the other would suffice. the ratios would be in percentage, ppm or moles of each. the latter is normally applicable when the elements form part of compound(s) especially since K+ and Rb2+ (you should evaluate Sr2+ also should it be present). So when you know the matrix of the sample, then further explanation would be clearer. Please let me know.

Correction!!!!!  left out .. the ratios be the percentage or ppm of each divided by the other. hence the ratio is dimensionless. when using moles as the concentration then it will give insight into the compounds/substances that contains these elements. Sorry for the (too fast) typing.

Thank you very much everyone. @ Dr Johan André Mars. The concentrations are in ppm.