On center or Off center mount

Ready to cut out and glue up transom for 2 outboards. Main engine is 60hp
and kicker is 9.8 hp both 4 stroke 20" legs. The hull is 8'wide x 28' and the aft 10.5' is a flat dory hull V ing as you go forward to 3/12 then to 6/12 then to 9/12 pitch 3' aft of the bow.
The 60 is 16.4" wide with a 16.1" swing each way. The 9.8 is 14.4" wide with a swing of 13" each way. Boat will be used for long river cruises on US rivers and lakes and is a live aboard for several weeks at a time.
Should I center the 60 hp and mount the kicker off to one side or center both based on 1/2 of the combined weight or some other formula?
Both engines will have equal use as I am in no hurry and have to watch my fuel use. Want best fuel usage and and power usage (performance ) and controls are mounted inside the cabin. Never have owned outboards before only inboards. Thanks much for any assistance. Forgot to mention the transom tilts 14* a 3/12 pitch.

 

I would install the main motor in the center and put batteries on the opposite side of the kicker for balance.

 

How about mounting the kicker on a raise/lower bracket on the back of the main OB so they are both on the centerline and steer by the same mechanism?

 

You don't want an outboard running behind any kind of obstruction, I'd be thinking of using your dead main engine to steer when the "kicker" is supplying the go-forward, and have it locked into the dead-ahead position. Obviously it won't work for tight manoeuvres, but for general travel it will.