To change to cavity doors you will need to remove the wall each side of the doors to accommodate the cavity housing. Normal sliding doors would make a difference but the problem with them is they never stay totally closed and they do become quite noisy and annoying. This problem also applies to double cavity doors. The other option is to use bi-fold doors, they won't intrude into the room as much. Do you really need two doors there at all and is the glass above the doorway necessary? My personal preference would be to fill it all in and just have the one door to the room. You could even re-use one of your doors.

Wow I too have the same glass above my doors. Mine are all internal swing doors therefore I don't have your delema. I'm in the middle of renovations and mine are staying. My walls will be dark and modern with all the trim white with the original character and it looks great. With your wall, it's a lot to deal with therefore would suggest replacing the slider with a glass door with timber trim, keep your walls light modern and bright and paint your trim dark (any colour you favour) here is my example to give you an idea of what I'm talking about hopefully you can see the value here is a snippet of my renovation good luck....hope my idea gives you plenty of ideas

If your block is facing North or West (within Australia) maintenance will be required to ensure movement is minimized. Timber doors are beautiful they just tend to be susceptible to sun damage. FYI Cutek is a great timber protector if you want to keep the natural look however you will need to keep applying year after year.

Oklouise and Sirius, thanks for the ideas. Actually the first one could work us very well, removing part of the wall and having a partition instead of a door. A lot more work but we are pretty handy. Regarding the barn door, most of the people I talked to agreed it might date quickly, unless I use a different mechanism than the ubiquitous black one you see everywhere,maybe.