2nd Thunderbolt iMac as display and time machine drive?

Hi I'm lucky enough to have two 27" imacs on my desk - the older one is a Mid 2011 thunderbolt machine, the other its replacement is a mid 2014 thin screen device.

I'm using target display mode to use the old machine as a second monitor, which is great, but a bit of a "faff"...

When I lock the screen of my "host" mac, the screen of my target display mac goes blank, but when I come back to my machine I need to unlock BOTH macs, (meaning I need two keyboards on my desk), then I need to do the whole CMD F2 jiggle thing and inevitably adjust the brightness on my target display machine as for some reason it always comes back too dark.

Secondly, while I can see the hard drive of my target display mac in finder, I can't use it as a time machine backup drive - I can no longer access my backup drive as it's firewire only, so I'd really like to be able to "flatten" the drive of the old machine and use if for backups via thunderbolt.

My new mac is basically a mirror of the old machine as I updated it using migration assistant, so I no longer need any of the data on it, - I don't really NEED the OS - what I would like is for the older Mac to simply be a "dumb" screen and hard drive which I don't need to boot up or anything is this possible?

(Apologies by the way if this post seems very familiar - I tried to post earlier today, but my message doesn't seem to have shown up - is there a moderation delay on here?)