Trying to speed-up render times on a large single-frame Cycles image by using netrender and a spare machine. The spare is running (in separate terminals) a master and a slave, and the main computer is running a client and another slave.

Pressing `F12` from the client works to render the image, however only the slave on the client machine is used. Killing that slave will then cause the slave on the spare to be used. It does not appear possible to have them work in concert. Is it correct to conclude that netrender only divvies frames to slaves, not tiles?