In bash, what's the diff between [[ test1 && test2 ]] vs [[ test1 ]] && [[ test2 ]]

What's the difference, if any, in the following two bash evaluations:

if [[ -s $file1 && $file1 -nt $file2 ]]; then

if [[ -s $file1 ]] && [[ $file1 -nt $file2 ]]; then