MWD....I am in Jim_MI's camp on two down riggers on a PA-14.....I have a Canon Mini-Troll on my 2013 PA-14 and caught two nice German Browns last week while down rigging at 30' and 50'.....a 21" fat fish does move the PA around.
Depending on where the fish decides to go and while reeling in the fish, if you need to rewind cable, a single down rigger is plenty to deal with.
In my minds eye, with two down riggers I see a "Chinese Fire Drill" in progress....
Then there is the question of location and "bridging across the cockpit" with an aluminum plate......I assume it can't be too wide, otherwise the mirage drive pedals might hit it.
For me, I know reaching the Big Johns down rigger where you are thinking would be an issue....all the way forward means getting on your prayer bones to do anything....just my thoughts.
It's good you are asking very good questions before taking the installation plunge.