That only works up to a certain degree, of course. You can't expect to do length or girth only and to gain in the other department respectively. But you will achieve some girth with length only (mainly base girth) and some length with girth only (due to the balooning effect), especially in the bginning of your Pe career. Then there's the jelq which is usually applied at least in the initial phase and that works both dimensions better than any other exercise (though it does neither as efficiently as more intense exercises do but it's the best allrounder, so to say)