How to isolate the 2 lists of ways for each driving direction in a Motorway relation ?

You could try ignoring relations, and extracting "WayChains". A chain of ways that are connected end-to-end.

My WayChains TIGER fixup analysis does this kind of thing (on motorways as it happens, although it would work for other kinds of end-to-end connected ways)

The logic to join together ways into "waychains" can be found in waychainsummary.rb.

I'm running it with a pre-processing step using my osmosis simplify plugin, which reduces ways down to just their start node and end node.

Note however that I was deliberately designing this to help find problems in the U.S. motorways data. For extracting OpenStreetMap data for practical use, you may find this approach a little fragile. e.g. if a mapper tags an onramp with highway=motorway instead of highway=motorway_link, then it may not chain the ways correctly at that point. You'd have to fix it before trying again.

I'm not that an easy solution exists. Maybe you can code (for example with python and DOM) an analyser on the data that, starting from a node, build the set of ways that are connected.

The best solution would be to split the relation into two parts, one for each driving direction. In the US, directional roles have been used: north/south/east/west, but this does not seem to be the convention in France.