Something like
Code:
{identifiers:'ifempty(select($, 'asin'), select($, 'isbn'))'}
should work, where you change the identifier names to be what you want. The first named identifier (asin above) takes precedence over the second (isbn).
To help you be able to do this yourself in the future, the above is an expression in
Template Program Mode. Here is the
documentation for the ifempty function and the
documentation for the select function. All of the
function documentation is here.