There are a few hack-ish options out there, see here, and here.
But I certainly wouldn't do it. This is a hack, it's not supported by apple in anyway and there is no guarantee that the next OS X update won't break it and if it does you're stuck with your backups in a network share that is pretty much useless at that point; perhaps if you only need the initial backup you'll make do, but remember that time machine backups are incremental, you are very likely not to be able to restore the latest version of your data.
With some bash $voodoo you might be able to pull it off (getting the initial backup, looking at time stamps, merging...)
From my point of view you have two options: you either stick with the apple supported solution, may that be local drives for time machine or investing on a time capsule, or you find another way to backup your two macs to your linux server avoiding time machine all together.
and on that last note I know a lot of people who swear by superduper, and for many years I've used iBackup which is in reality a glorified GUI for rsync and that gives me some comfort.