After looking at my original picture, I noticed that it would probably be hard to understand what I mean, please take look at picture attached to this post.
Slidehow solution wouldn't work for me because I need to switch between images inside k2 item, not between k2 items.
1. I have a product, so I create a new k2 item.
2. I add main product image using k2 image tab.
3. Then I add new gallery using k2 gallery tab and sig pro.
When I open this product I have 1 main image (from k2 image tab) and a sig pro gallery right below it (just like in attached image)
When I click on main image (from k2 image tab) it opens up in joomla modal lightbox, and I can't switch to sig pro images, I have to close main image and then click on sig pro images in order to continue viewing products images.
As I mentioned earlier I almost achieved what I want by manually adding
to k2 item's image in k2's item.php
But unfortunately I don't know how to output
automatically in k2's item.php using
rel="prettyPhoto[gallery<?php echo $gal_id; ?>]
If there would be a way to output sig pro gallery id inside k2's item.php, my problem would be solved.