Is it possible to render a single image using multiple computers on a network ?
Ye, this is an essential feature of every renderer. Either use CoronaDR for multiple PCs for one image or backburner for animations.
And even better, using the progressive preview interactive render with multiple computers ?
This is currently not possible and has a limitation, in that the netwroked PCs need to redownload all textures and max file to start rendering with each change promting a redownload of the .max file aswell. This will put stress on I/Ops and networkcard of the workstation and will take time to implement and figure out. Also this goes a bit against the philosophy of a fast, draft preview, since it will take time for renderers to start working. You can suggest it if you want, with a proper feature request.
it's in "Resolved feature requests" Does this mean it's available ?
No, the wording is a bit unlucky here. It means "We have read the suggestion, wrote it on the white board and now delete this so it doesn't spam the suggestions subthread, since it's already acknowledged"
This feature would be actually possible to implement in a short time, since the hearth of the feature, resume render is already coded, but the priorities lie somewhere else currently.
And hell! you can do it yourself with overlay in post. Might post a guide on that.