You're not supposed to assume both containers had the same amt of gas to begin with. The volume of each container doesn't change. The total amount of gas doesn't change. So there aren't that many unknown variables at all! As I said, use PV=nRT for both containers. Lemme do it here:
Let f denote final.
[tex] P_1 V = n_1 RT_1[/tex]
[tex]P_2 V = n_2 RT_2[/tex]
[tex]P_f V = n_{1f} RT_{1} = n_{2f} RT_{2}[/tex]
This should get you started =)
