Generally speaking ( that is, with a Pokemon that doesn't have abnormally large HP ) you should always invest 252 HP before even thinking about the defenses. Therefore, your optimum stat spread for taking hits equally on both sides of the spectrum would be 252 HP / 128 Def / 128 SDef.
However, if you have a positive nature on one of the defenses, then the EV spread closest to equal will be 252 HP / 80 Def / 176 SDef with a Defense boosting nature.
A Mew with 252 HP / 128 Def / 128 SDef will have 404 HP, 268 Defense, and 268 Special Defense.
A Mew with 252 HP / 80 Def / 176 SDef with a +Def nature ( eg Impish ) will have 404 HP, 281 Defense, and 280 Special Defense.
I'm not sure what you mean by your last question; but as far as I know, if you have a Pokemon with equal defensive stats ( like Mew ) investing EVs equally will always make the actual stat equal too. Investing in HP first is the general rule of thumb though.