Getting spare shaft for the D44 have a ?

I have the XJ D44 rear I know the shafts are different lengths but can I just get one that would work for both sides right now or do I need to get both?
I plan on getting the other when I get the funds a month or two but I want one spare now my real question is if I break the long side can I put in the short side shaft? Also what is the short side and what is the long side? If I cant swap what one is the most commonly broke? I am welded in the rear so lots of stress. Thanks
And Keep DIGGIN IT!

The difference is only about 3/8",so a short axle would work on the long side!