You have two options, I think. One is to buy ANOTHER 13 pony and rig it slung like a stage bottle. It is safer, won't cause problems for the cattle boat tank racks and should be somewhat less disruptive to your trim.
Another option is to buy a 6 cu-ft pony and tank mount it. This is what i use often in WPB in the typical wreck and reef depths. If you are petite, and your air consumption is commensurate with your size, then you may find this has enough capacity to get yourself to the surface with no time to play around. This tank is so small and streamlined that it should provide no problems - I had my kids using them at 9 yrs old without trim issues. But you need to do the math and determine if it will give you enough time to "not die". In all honesty, I am more comfortable with, my son (who is 18 yrs old now) wearing a 13 and me wearing the 6 cu-ft, than vice versa. Do the math, balance convenience, weight, drag, safety and make a selection you can live with.
WPB drift diving with insta-buddies is pretty ridiculous because when the current is running, a 2 second separation in the wrong direction can COMPLETELY eliminate the possibility of a buddy rendering help - so some redundancy makes sense for me.
In other words, if you find yourself upstream from a buddy, in 2 knots and sand separates you, the buddy may be incapable of traversing the sand in an upcurrent direction. If you are stuck or entangled on the bottom, you better be able to handle it on your own.
Lastly, a back mounted 13 shouldn't really be that big of a deal, it should be mounted on the side and as far forward as possible and up against the backpack or BC, this will reduce the tendency to roll you sideways. If it doesn't work for you, find something that does.