Try pressing [SHIFT + M]. After creating one Radial gradient, [SHIFT+M] makes a new independent Radial gradient mask. But does not 'copy' the shape of the first mask. As you find - a Copy & Paste [CTRL+C] [CTRL+V] creates a mask copy that is linked in adjustments.
A [Right-Click] can Duplicate the mask (same size but independent) - I have not yet learned if there is a keyboard shortcut for this! (ie. same size, independent adjustments)