I dont know if there is a way to render backwards, BUT !!
if you set the same render output path to some network directory for every machine and tick/check Placeholders and uncheck Overwrite you can render with both.
it works like this: if a machine starts a frame it creates a placeholder file and if the other machine tries to write the same frame it can not because it thinks the frame is allready there, so proceed to next and so on.
for example Machine 1 starts Frame 1, creates a placeholder and Machine 2 starts with 2.
now - 1 is finished and 2 is working
Machine 1 wants to render Frame 2 but Frame 2 is already (placeholder) in the folder it continues with Frame 3 -and so on.
everytime a machine finds a placeholder it skips the Frame and continue with the next