About Analog input

I have two presseure x`mitter . I connect one at the start of the line and another at the end of the water pipe line . Both x`mitter is giving me 4-20 mAmp . I have only one analog input in PLC and i want to measure differensital pressure by using this two pressure trnasmitter . Please suggest me how i wired both transmitter that i will get 4-20mAmp as differential pressure .

By:-

Dhaval Patel,
Gujarat, Surat .