I have a dual spinner panel. Check it out on my web page. I don't really know that it is an optimum layout. I just basically put one spinner to the left of a top firebutton joystick for games like tron and the other spinner I just put where there was room for it. One spinner is connected to the X-axis of the encoder and the other is connected to the Y-axis of the encoder.
Dual spinners are great for multiplayer games like Warlords, Offroad Racing, and the Sprint games; but ... the official version of MAME doesn't support this. MAME variants like Analog MAME, EMU+ (discontinued), and others allow you to map Player 1 to the mouse X-axis (spinner1) and player 2 to the mouse Y-axis (spinner2). I have no idea why the MAME devs won't put this functionality in the official MAME.
My hat is off to all the great people who selflessly devote their time to the MAME project, but I just wish they would stop adding games for a while, fix the bugs, and add in some simple control features so that we don't have to download all these customized versions each time a new release comes out.