Circuit with 2 reed switches (proximity sensors) output to single GPIO
I think using 2 digital inputs is the way to go, but absent that, you want to build a 2-bit Digital to Analog Converter. 



This pic, from http://tabatha.esg.utulsa.edu/digital/pages/r2r-alt.gif, shows a 3 bit converter.  You would simly omit the bottom bit, leaving off the 2K resistor at V1 and the 1K resistor attached to it.

You won't get your table, but you will get unique voltages for each bit.  For this 2-bit converter, if your switches provide 5V, one bit will add 2.5V to the output, and the other will add 1.25V.  So, none on would be zero volts, if the switch attached to the LSB is on and the other off, you'll have 1.25V, if the switch attached to the MSB is on and the other off, you'll have 2.5V, and if both are on you'll have 3.75V

Your issue with this approach will be implementing it with SPST reed switches.  You may need an op amp buffer on each one.