Two spears prevent the bale from spinning on you when you pick it up.  Also obviously doubles the carrying capacity, if the same spear is used.  Spears on the top of the frame give you a bit more reach if your loader is marginal for the top row of bales.  If the loader has plenty of reach, spears at the bottom eliminate the frame hitting the net on the bales underneath while grabbing the top one out of the cleavage. We usually run them "up" on the GB800/4010 and "down" on the Swartz(GB900 clone)/4630 or the telehandler.   Our quick couplers on the tractors allow them to be simply turned over and coupled on the other way. 

We built every bale mover we ever had up until about six years ago.  We bought some pointy, supposedly forged spikes then, because we had started having trouble with the ones we built not being strong enough for ton+ bales.  The store bought ones turned out to be worse.  The absolute best ones we have had were made from the stems used with a pipe pusher jack that pushes pipe under a roadway.  Old A-C WC or WD era toolbars are pretty good, too.  They don't bend, but they can still be broken if you try hard enough, especially when it is colder the heck.  

The #1 way to bend or break them is to stick them in the ground while the tractor is going forward.  That's probably the #2 way, and the #3 way, and the #4 way, also.  

The socket mount is kinda nice if you have "factory built" spears that bend every time you use them....you can just turn them over and they will bend back the other way again.  Really, it would be a lot handier if they just didn't bend to begin with.  

Keep the spears close enough together that you can stick the bale above center without getting into the net.  The bale is much stronger above or below the center...if you stick it right in the center you will be in the part where the stems are going up and down and it will tend to sag a little.  It's also usually a good idea to stick the spears in a little bit downhill, so that they can 'claw' the bale a little as you curl and hold on better.....we used to unload with the neighbors 158 with just manure tines on the bucket.  You stuck them into the bale with the bucket curled all the way up, and they would work about like a hay hook on an idiot brick.  The same thing works moving rounds with the square bale mover which has four short spikes...you just have to make sure you give yourself enough room to curl enough to bite into the hay good.