restore /usr dump on two hard disk parallel y

[s m]

hello guys
i'm trying to restore DUMP file for partition /usr on tow hard disk
parallel y. these two hard are connected to my system (i have freebsd8.2).
i use  restore command and it uses /tmp directory to restore dump. in
restoring dump process, two hard disks try to use /tmp directory of my
system. therefore conflict happened and restore command return error.
i try to use TMPDIR and define another tmp directory for one of my hard
disk but it does not identify it and use my system tmp directory yet.
please let me know if using TMPDIR is a good idea and how i can use it. if
not, how i can restore /usr dump file on two hard disk parallel y?
thanks in advance
sam