NOTATIO

I have a score with 3 movements. The last measure of each movement blends into the next movement so that all 3 must be performed to make musical sense. If the players want to play any of the movements as a stand alone, the last measure of the of the selected movement must have a different ending. Basically each movement(except for the last) will have 2 different endings.
1: if the players play it as a stand alone.
2: if the players play all 3 movements together.
I remember seeing this in a score once but I can't remember how it was engraved.

You might have the two endings bracketed like a first and second ending, but instead of 1. and 2. have: "For continuing" and "For ending".

I second John's suggestion, and I would just add that you should, of course, clarify your intensions for each bracketed ending by supplying it with the appropriate double or final barline.

I agree, and I would put “For ending” first and “For continuing” after, with a gap between (very effective in Sibelius).

Thanks. I tried to look this up in Gould's "Behind Bars" but couldn't find a reference to it.

I had a whole suite to engrave for a customer with this approach two years ago. Have a look.