I still don't think the concat filter has made it into the official Shopify docs (and it might not ever) but there's a little clue of its existence in this file. Concat is probably my fave Liquid filter to date - it's awesome and quite useful for simulating a push filter.

Being undocumented comes with a level risk in use within Shopify - and if you did use it - still keep efficient code and memory usage in mind.

To put this into a simpler example let's just create two basic arrays and join those suckers together into one. 

{% assign arrayOne = 'a,b,c' | split:',' %}
{% assign arrayTwo = 'd,e,f' | split:',' %}
{% assign foo = arrayOne | concat:arrayTwo %}

So in that little example, foo now has the contents of both arrayOne and arrayTwo. Awesome stuff! Now you can loop over that array just like any other. What's really cool is that this will work for products, blog posts, etc. Far simple method of joining things together than needing to resort to lengthier capture string and split hacks. 

[update] This is no longer an undocumented filter. Jump to the official help docs.