Can't you just make it that there are 2 different autodemo "slots" (like autodemo1.dem and autodemo2.dem. Yes, creativity rocks), so that the first map is stored in autodemo1.dem, then the following one in autodemo2.dem. So you could still save the first demo without having to fear about that it's going to be overwritten. For sure the option to manually set how many "slots" you want to have would be the best.
In my opinion that would be by far better than the timelimit it has now. And it also doesn't really seem to work anyway.

How about you just press F8 at the end of the map, so it saves the demo of it? And if you set the timelimit to 0, it saves the whole match until you press the stop button, if I'm not mistaken.

I did use F8. Though the problem is, if you forget to press F8, the demo is wasted. Also it worked very differently for me. For example I saved after a map, and it showed me what happened BEFORE the map. Hm?

What if you forget to save after two map changes then autodemo1.dem and autodemo2.dem will get overwritten?

Yes, it would just switch to autodemo1.dem again. But this way the whole demo wouldn't get overwritten just because you forgot to press F8. If you'd have 3 autodemo slots it would switch to autodemo1 again after having recorded the third one. If you have 4 slots... you know.
(btw, I didn't know about that you can set the timelimit to "0" in order to disable it at first, which makes it a lot more useful the way it's now for sure)

Id actually prefer if there were 2 autodemo options available:

1) the current one with maybe 2 or 3 slots as mrx suggested
2) continous saving of demos, like it works with screens (mabe deactivatable via a shortcut)