First off, do you mean 2 private servers, or private server with NFNA?
For the former, you can either just run the appropriate .exe that changes your hosts file each time you want to switch servers.
For running a private server with NFNA, you will need to change from Chinese PRC to English as your non-unicode language each time which, unfortunately, requires a reboot. >.<
ok i think i can help you out. if you want to run 2 on 1 pc ( i take it for trading for grinding low lvl's) whatever anyway download the program sandboxie
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now you run nf first in sandboxie then you tab out and run normal from desktop or whatever then you can alt tab between the 2 hope this helps.
VMWARE!!!!!! Install a virtual xp os on vmware and install navyfield to the new virtual operating system and presto can run multiple accounts. This is my theory at least