Yes, I only had two bobbins and I just reused them adding the color of thread I needed on top. Now we have so many store around that sell sewing supplies that it is hard to imagine there weren't that many places to buy equipment for your sewing machine.

I do it all the time especially if I just need a small amount of a certain color some of my bobbins have 3 or 4 colors on them.

My old Singer only had one metal bobbin, so I guess I probably would have been guilty of that.

I remember my mom doing that so she would not waste the thread or she would take a little scrap of paper and wind the remaining bobbin thread on that and use it for hand stitching.

It is amazing what was done not to waste anything. I haven't put 2 colors of thread on 1 bobbin very often, but I do rewind it back on to the spool if it is a color I don't use very often & there is quite a bit left on the bobbin. I have a wonderful machine guy that gives you extra bobbins when you go back into the store just for the asking. I never seem to have enough bobbins & always looking for an empty one.