I used to work in this field so maybe I can shed some light. In the case of co-witing i.e. Jagger/richards, Tuapin/Elton John etc... Both parties had a hand in the creation and therefore get a split. What the split is, is determined by a contract that is on file. Writing credits also called publishing are paid to the writers at a standardized rate (in Canada I think it's 7cents a minute per song, per copy pressed.) for every time the song is reproduced. These royalties are collected by a private agency like the Canadian Rights and Reproduction Association or by the record compnay themselves and then paid to the writer. As for who gets paid when the song is played, that is different. That is called performance rights and are collected by SOCAN and distributed to the writers/holders of publishing. The players on the recordings themselves do not get paid beyond the original session payment unless they had some tyoe of special contract. Basically studio playes get paid large one time sums, or a % of sales if the negotiate well, or can negotiate for a % of the writing credit but unless they do this in advance in writing they do not get paid beyond the original session unless they wrote it. It pays to be the writer.