In general, I think it inadvisable to apply for a subject and talk about a different subject in your PS. What subjects do you want to apply for? Can you find no way to write a PS which incorporates both?
I applied for a mixture of Norwegian/French/German and I got offers from everywhere even though I hardly mentioned German and I didn't mention French at all. If you can find common themes which are relevant to both subjects then it is possible to write a good PS, for example, I talked about linguistics and the status of minority languages in Europe.