Change Quantity of multiple products and add it to cart
This should be possible. What I would do (from the bottom of my head, other alternatives can be easily implemented) is described below.

Assumptions:


There is an entity reference field to indicate relevant products.
There is no relation of 'additional' products back to 'main' or other products (the additional products are refered to by main products and they have no other reference to any other).
You have basic Views knowledge
You have basic Rules knowledge
You have basic Views Rules knowledge


Method:


Create a rule on event 'add an item to the cart'
Condition: the added item has reference to 'additional' products (so this is not an additional product).
Redirect to a view page which presents all products that are relevant to the current user's cart items (a view on order items entities).
The view should have an add to cart button without quantity
Create a rule on the same event 'add to cart'
Condition: the added item has no reference.
Loop through all cart items and call a rules component.
Create a rules component with 2 arguments (item added, item from loop) and 1 condition: if item from loop has reference to the item added), return the quantity.
Set the item quantity to the one returned by the component.
Optionally redirect to somewhere else.


Mind that the condition needs a bit of care. Alternatively, you can avoid Rules altogether and create a custom module which will use the Drupal Commerce API.