Need help placing a second subwoofer.

I always place them in stereo pairs behind and outside (if possible) the main speakers. This significantly extends the soundstage. You want the arrival times to reinforce the main speakers.

"...I always place them in stereo pairs behind and outside (if possible) the main speakers..."


Same except I put them on the inside.

place any subwoofer or subwoofers where they sound best as a first effort.  consult the owners manual.   making a nice "stereo pair"  looks nice and tidy but you may be wasting their potential.  usually subwoofers are recommended to be placed near corners for best response without getting too close to the walls to cause peaks in the response.  
behind the listening area is certainly acceptable as long as you refine the position by testing.  too close to walls you will hear an exaggerated peak (one note louder than the others, not good)  
pick your spot and then listen to see how it sounds.  too weak = too far from wall boundaries, too over powering, too close.  usually 12 inch from walls minimum. 

10" is not lame, you just need more of them. https://systems.audiogon.com/systems/8367 

The thing with subs, placement with the first one is time consuming and ultimately impossible. Because being only one, then no matter where it goes you get great big peaks and dips. The time consuming part is moving it all over the place before finally giving up and living with huge compromise, telling yourself it is really good anyway. 

Second sub placement is a bit easier. Now with 2 subs each one puts out less volume so the peaks are smaller and so are the dips. All you really have to do is put the second one somewhere a different distance from a corner than the first one. This will make the peaks and dips as different as possible from the first and give a more smooth bass response.  

This is where most of the improvement comes from, the smoothness. That is why it matters much less how good a sub you use. It is much more important to use a lot of them. With each additional sub the bass becomes even smoother, faster, cleaner, deeper and more powerful. Each additional sub lets you turn down the volume on the others. So you also wind up with a lot more dynamic headroom. Plus placement gets easier and easier. With four you can pretty much put them one to a corner different distances and be done. That easy. 

"...making a nice "stereo pair" looks nice and tidy but you may be wasting their potential..."

OK, here's the deal. In most cases you want the subs crossed over at higher frequencies, say 80 HZ. As you tune higher for better response, the sub can be located. So if you put them inside it becomes part of your stereo and helps with the imaging. If your subs are for HT explosions, that is a different deal.