If you try learning both at the same time, chances are you'll end up speaking portuñol, haha. Believe me, you don't want that. Besides, learning portuguese once you already have a good command of spanish is super easy, whereas learning both languages at the same time -given that they're very different from your mother tongue- is just going to make the whole learning process more confusing and difficult. I believe it might even take you longer to achieve fluency if you try to learn them simultaneously rather than if you tackle them one at a time...I really wouldn't advise it but it's up to you :P