There is nothing wrong with drawing fuel from both at once, but the problem is getting them to draw evenly. One tank will be more "dominant" than the other (goes empty first) and when it does, your motor will quit, even if the other one is full. Then you have to pull out the vice grips to clamp off the line to that tank to get it restarted. I like a tank selector valve myself, but if you want to run your setup, figure out which one is pulling faster, then put a valve on it to cut it down, until they draw even.
Have Fun!