I think curtain track is not symmetrical, at least not the ones I have seen.
Is the 1200mm the full length of the finished item or the total stock requirement?
If the finished work is lots of short pieces, and depending on exterior appearance, it may be possible to make each piece by slitting a length of square tube into two and soldering them back-to-back. Or better, if available, brass channel of appropriate size.
Distortion might be a problem though, unless heated very evenly.
I used something like that principle to make the H-section beam front axle for my steam wagon, but that is of larger section, steel, and welded. The stock was folded channel.