i tried 2 ways but i got different results
1) ctrl+mouse click on A channel , shift+ctrl +click on B channel -> save
You won't get valid results by addition of channel components. L*a*b* does not have linear colorants that can be simply added together. (In fact no colour models do, but Lab even less so).
You mean a mask that is the sum of the a+b channels?
You can try making a copy of the a channel, and use the "Apply Image" command to apply the b channel on it, using one of the "contrasty" blend modes (Overlay, Soft Light, ecc...).
This is not scientifical, and note that if you apply a over b, or b over a, you are going to have different results..
but i can sum alpha masks in rgb mode ?
i see , i tried and i got different results
You can load a channel as a selection by ctrl+click on the channel thumbnail. And if u add the shift key you can add another selection.
The point here is that adding a+b make no much sense "as a selection", because they have 50% grey as "zero point", instead of 0 (as a normal alpha channel, aka your selection, has). So it depends on what you need to do!