I'm planning to keep the second mirror which is c3t1d0s0 to have a healthy copy of the OS and I'll detach it and I'll mirror it every week one time to be healthy OS copy for me and fast roster backup for me if any thing happen for the first mirror.
which is mean it'll be all the week detach and in the end of week I'll attache it to have full mirror then I'll detach it again
is there any problem to keep my system running with one mirror and another one will be mirrored every one time every week ?
In principle you can do that, because mirrors are meant to work even if one part of it is missing (otherwise there would be no point in mirroring, no?). On the other hand you will run the whole week with only one mirror and if something happens to this disk your system will eventually crash, whereas it would continue to work with the mirror in place.
Do not confuse things: a backup is a device for data integrity, a mirror is a device for data security. If you want only the one and not the other you can do what you described but to have both you should consider having three copies one of which you detach and set aside as backup.
Btw., i presume it would be easier and less cost-intensive to backup the conventional way and use tapes instead of this rather tricky setup. You will have a longer downtime if your system needs recovery (restoring from tape is slower than to activate the backup disk) but tapes are on average cheaper than disks and you can even store several generations of backups, which is not possible with one disk - you would need awful lots of disk drives for this which quickly makes it unfeasible.