There is one circumstance under which this can work.
If you have an R2014a license for a product, then you are permitted to install and use any previous version of MATLAB under the same license. If your R2014a license is node-locked the install of the previous version would have to be on the same host as the R2014a version; if your R2014a license is floating then using the previous release would "use up" a slot just as-if you were running R2014a.
This backwards-permission is useful for testing code for compatibility, to be sure that your code will run in a previous release. It is also useful sometimes if you need to save a model or file in a format compatible with a previous release. For those of us who answer questions here, being able to refer to the previous documentation and test against previous releases is also important so we can see exactly what the other person is seeing.
It is, however, not possible to use a later toolbox with an earlier version for the purpose of accessing features not available in the earlier version. When you were using the R2012b version all of it would be R2012b.
It is also not possible to "lend" a license to someone. For example if you have the toolbox on your R2014a version and you are using a Named User license, then someone else cannot use that R2014a toolbox license on their R2012b installation. If it were a node-locked license then it might be within the terms to allow someone else to sit at the keyboard to use the license (it would depend on the exact form of license you had), but for a Named User license it would not be allowed to allow someone else to use the license under your name; you would have to do the work for them (yes, they could be telling you step by step what to do, but it would have to be you at the keyboard.)
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