The 2 gas  are separated by a closed valve

the ++   is the valve
    ||

What happen to the pressures then the valve is open ?


(-----)      ++    (------)
(     -------| |----      )
(     --------------      )
(-----)            (------)
some gas             some other gas
V=2.5L               V=1.5L
P=2.33 atm           P=1.65 atm