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Have the use of these two laptops:
MacBook-Pro Retina (2012).
MacBook-Pro Retina (2014).

Is it possible to use one in Target Display Mode (& drive) from the other?
Presumably via a ThunderBolt Cable.

No guidance here:
https://support.apple.com/en-us/HT204592

No. Only iMacs support Target Display Mode. 27" 2009/2010 iMacs over Mini DisplayPort, 21.5" or 27" iMacs mid 2011 - present (excluding the 5K) over Thunderbolt. That is it.
 
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