Yes. You can have multiple partitions and OSes on different parts.
My working drive I use for jobs has 13 partitions on it. Installers from 10.5.8 
through to 10.12.1, then bootable drives in various flavours. Then a “spare” 
part for other things. (updates, image files etc).
As a note, only some machine will boot certain systems. But those two should be 
fine.

Kind regards
Daniel

Sent from my iPhone 7

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Daniel Kerr
MacWizardry

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Email: <daniel AT macwizardry.com.au>
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> On 25 Nov 2016, at 1:15 pm, Matt Falvey <[email protected]> wrote:
> 
> Hi I am looking to partition a portable external hard drive. 
> 
> I am wondering if it is possible to put two different OS on the partition? 
> (OS X 10.9.5 on one half and OS X 10.10.5 on the other.)
> 
> Thanks
> 
> Matt.
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