912 is Fuel Selector

Keep it simple.

There is no need to be able to select individual tanks unless there is an actual "Need" to select individual tanks.
If you select a single tank with a 912is you will also need to return the fuel from the Engine's Fuel Pressure regulator back to the same tank that it came from.
This will require parallel fuel selectors so that both the supply and the return lines switch at the same time. $$$

Plumb both tanks together with a generously sized hose and the two tanks will cross flow and equalize themselves. A simple ON/OFF fuel switch will be all that is needed.

The fuel does Not slosh to the side of the tanks when maneuvering unless you are Skidding or Slipping in the turn. You can make a coordinated turn with 60° of bank and the fuel will stay level in the bottom of the tanks.
If this were not the case, then it would be necessary to be constantly selecting tanks whenever you made a turn.
If you are going to go through the expense of installing a Left/Right/Both fuel system and then leave it in the Both position all the time, you might as well just plumb it as a BOTH system and be done with it.

Two cross-connected tanks feeding a simple ON/OFF valve worked quite well in the over 30,000 Cessna 150 and 152's fitted with this arrangement.

Did I mention, Keep It Simple!

Starting to work on the fuel system for our project.  We plan to use the Rotax 912iS engine.  Two vented wing tanks flow into abt a 2.5 gallon header tank.  The header tank has a outlet at the bottom and a fuel return inlet at the top.  My partner are trying to locate a good fuel shut-off switch/valve and haven't been able to locate a suitable candidate.  We've looked at the Andair and Newton offerings from Aircraft Spruce.  Based on the Rotax installation manual we should be shutting off both lines, supply and return.  Has anyone found a good ganged shut-off valve?  Don't need the Right/Left/Off offered by the Spruce valves?