Hi!
I'm a Maya 2013 user and I installed Houdini Engine and it works great!

The problem is I have both Maya 2013 and 2015 installed. And when I installed houdini engine, only my Maya 2013 recognizes the plug in.

I'm currently testing new pipelines so I really need to have both Maya running Houdini engine.

Is it possible?

I'm guessing HouEngine “found” Maya 2013 paths&folders first and installed it..

Thank you for your time!

The plugin files are actually installed. It's just that the installer didn't find Maya 2015, so it didn't configure Maya 2015 to find it. The plugin files are in the Houdini installation directory:
$HFS/engine/maya

You can manually configured Maya in two ways:

1. MAYA_MODULE_PATH environment variable
You can set the environment variable through Maya.env file or through your OS. If you use the Maya.env file, you'll need to add a line like this:
# Linux
MAYA_MODULE_PATH=/opt/hfs13.0.516/engine/maya:$MAYA_MODULE_PATH
# Windows
MAYA_MODULE_PATH=C:\Program Files\Side Effects Software\Houdini 13.0.516\engine\maya;%MAYA_MODULE_PATH%
# Mac OS X
MAYA_MODULE_PATH=/Applications/Houdini 13.0.516/Houdini Engine/maya:$MAYA_MODULE_PATH

2. Maya Module File
This is what the installer does. There should be a Maya module file in the Houdini installation directory:
$HFS/engine/maya/maya2015/houdiniEngine-maya2015
It's a text file that contains the full path to where the Maya plugin is. Something like:
+ houdiniEngine 1.5 /opt/hfs13.0.516/engine/maya/maya2015
Copy the file to the modules directory inside the Maya installation:
# Linux
/usr/autodesk/maya2015-x64/modules
# Windows
C:\Program Files\Autodesk\Maya2015\modules
# Mac OS X
/Applications/Autodesk/maya2015/Maya.app/Contents/modules