One SQL Server. One SSIS. Two versions of DTExec.exe? Is it possible to install and use the 2012 version of the DTExec.exe on a Server 2008R2/SSIS2008 box without installing SSIS 2012?  All jobs will be called from the command line, no server agent jobs. I see that the 2012 SSDT BI Tools download contains the DTexec.exe tools and it's required binaries. Would that work?

An example...

The path to the existing 2008 DTExec tool on a 2008R2 SQL Server is.. 

\\SQLSERVERNAME2008\Program Files (x86)\Microsoft SQL Server\105\DTS\Binn\DTExec.exe


Could we drop the 2012 DTExec and binaries from SSDT BI Tools onto that server into this path?

\\SQLSERVERNAME2008\Program Files (x86)\Microsoft SQL Server\110\DTS\Binn\DTExec.exe


Will this work?  Do we have to install SSIS 2012? Why?  Why Not?  Any help you can provide will be useful.  Thanks all.
No.

SSD-BI edition is now a free download. That license allows you to develop SSIS packages that target a 2012 SQL Server Integration Services machine. That license does not grant you the right to use it for production-like usage. 

Furthermore, when you try to run a package from the command line, you're going to be met with an exception stating that ~ this is only for invocation through Visual Studio. The only way to make that go away is by installing the SQL Server Integration Services Service which is part of the 2012/2014 installation media.