There are several things that are important here. What kind of tape drives do you have? What is the total amount of data that is going to be backed up? Having only two tape drives I doubt that you are able to do a full backup of all during the weekend but again, that all depends on the number of files and the total size of it all. Might consider spreading the full backups from friday to sunday and doing incrementals the rest of the week. You must carefully consider the backup windows so that you are sure to get a copy of everything. 100 servers on two drives is alot, but should be possible depending on the speed of your network, your tape drive specs and general needs. I guess if you have one or more SQL servers their admins have set the maintenance plans to dump the database to disk at a certain time. So needless to say all backup windows of those servers need to start after the dump is complete. So that is a good place to start.