your body doesnt work best that way. here's why:

if you do cardio first, it takes your body roughly 20-30 minutes just to burn up the sugar in your blood stream, thus you arent burning any fat for the first 20-30 minutes of cardio(unless you do it first thing in the morning on an empty stomach in which case you would neevr lift weights in that session) so you ve basically wasted 20-30 minutes just to get to the point of burning fat. also, your muscles need that blood sugar to contract properly and efficiently, which they now can no longer do since that sugar is gone.

now if you do weights first, your muscles actually have the sugar to function properly, and after doing 30-45 minutes or so of weights, your blood sguar levels will be low enough that once you start doing cardio, you will virtually be burning fat from the getgo, thus getting leaner much faster and at the same time strengthening your muscle structure