Can I combine two heat-maps in QGIS?

Knightshound's answer is correct. Another option, if you don't want to create yet another file and keep ti always updated, is to use Virtual Layers. I'm not sure if Virtual Layers is available in QGIS 2.12 though.

• Click add Virtual Layer button

• Enter the following SQL query

select geometry from senders
union all
select geometry from receivers

• Click OK

Now you will have a brand new layer called virtual_layer, that you can use to create the joined heatmap.

Note that the virtual layer will be updated once any of the to layer are edited. Which is nice.

This can be easily achieved with 'Vector > Data Management Tools > Merge Vector Layers'. Does what it says in the tin and merges you two point layers to a single shape file. You can happily add a heatmap style or create a heatmap layer then depending which version you're using.