Remember to check your felt light traps before taping the two rolls together. Also many compact cameras use short leaders and if you are very careful when loading you can get a few extra frames. I think the initial leader that becomes the end of the roll when it gets rerolled would be the most wasteful part of this.

I have used a similar concept for shooting 35mm films in medium format cameras where you cannot rewind film and need that 2nd spool to hold your exposed film for removal.