okay so lets say i have an analog in vdc in 0-10v lets call this signal AIN
now i want to divide it to two analog inputs which well call A0,A1 in order to not lose resolution.
so input A0 will see the the 0-5v range of AIN as 0-5v.
and input A1 will see the 5-10v range of AIN as 0-5v.
i am planning to use an external adc with a resolution of 16bit.
but the voltage range limit of those i found are ususuly 5volts and in general i want a solution that will work for a higher voltage range than 0-10v.
*i dont mind if the solution requires a custom pcb or a cots item
You must not exceed the voltage your board runs at on the analog input pins - typically 5V, might be 3.3, depends on what Arduino you're using - you didn't tell us.
With stuff like that it's a good idea to add protection circuits.
Multimeters are usually made with voltage dividers (and protection circuitry). You lose voltage resolution at higher ranges but not percentage resolution (as long as the resistance errors are calibrated-out).
Just quickly checking the specs for my Fluke meter, the tolerance for most measurements is 0.5% to 1%, and +/- 1 or 2 counts.
What I would probably do is use a voltage divider on one input for the higher range, and a protection circuit for the 0-5V input. That way you keep the full resolution in the lower range.
You could use an op-amp summing amplifier and sum the input with -5V to "shift down" the voltage. It's an easy thing to build but you need + & - power supplies and with analog errors & noise it probably won't really help with resolution & accuracy. 
16-bits of resolution shouldn't be a problem but 16-bits of accuracy is "difficult" in the real world. There are offset, slope, linearity errors, noise and drift. (Most of those can be corrected/improved software but 16-bits of "perfection" is still difficult.)
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