If I had two new pieces, purchased at the same time, I would break them in together. How else better to have referent. When I'm upgrading, I purchase one piece at a time over a long enough time, so that I become familiar and very intimate with my system. Then, when it's time for the next piece, I am very clear on what I'm hearing from the new piece. I once purchased (got a killer deal, that I couldn't turn down) new ICs, and wouldn't (though I did, passively, break them in) hook them up for a listen until I gave my new integrated more listening time (about 5 days). It's the only way for me....
Some questions on break in periods....
I realize that some equipment requires a break in period before it sounds the way the manufacturer wanted the unit to sounds, so what constitutes as "break in?" Is it actually usage of the unit in question or does merely leaving it powered on through the night count as part of the break in? If the unit sounds bad out of the box, how many hours are required for break in before the listener decides the unit will never please them? One last thing; should a listener break in one piece of equipment at a time, break it in, before testing another piece of equipment? It seems to me that breaking in one unit at a time would help a person zero in on which item in the audio chain is changing the dynamics of the system.