It’s easy to merge two arrays with array_merge, but have you came across to merge two objects? It’s not that easy in Magento. You need to convert it to array first, merge them, and convert it to the object again to make it work. If you have two objects of different class, then it’s really difficult job to merge as both will not be compatible. If they are from similar classes or share same elements inside, then it’s not that tricky.

If you want to add collection2 in collection1 (in Magento), where collection1 will be a merged form of both, you can do so by:

foreach($collection2 as $coll) {  
 $collection1->addItem($coll);  
}

In core PHP, it’s quite easy in this way:

$merged_obj = (object) array_merge( (array) $collection1, (array) $collection2 );

In Magento, converting collection to array is easy:

$arr = $collection->toArray();