What I'm trying to achieve is when an item is created in a SharePoint Online list located in one site collection, to create that very same item in another SharePoint Online list located in another site collection having the same field values(that I managed to achieve). Then, when that item(the created from the first list one) is modified in the second list that took the entry from the first list, the field values that were updated in the second list to be updated in the corresponding fields for that very same entry in the first list. Any ideas how to achieve the second part? I used Microsoft Flow to achieve the first part. Pic of the flow that works only from list1 to list2:
Add a field in the first list, which holds an Id of an item from the second list. Let's call it "ID" field. Then when an item in the second list is updated, find the record using "ID" field in the first list and update that record with values from the second list item. You should modify your first Flow to additionally update the first list (update "ID" field when the item in the second list is created)
Does it work for you?
What Sergei has described will work. I have implemented a similar set of Flows including deleting an item in the secondary if it is deleted in the first. Make sure to Enforce Unique Values in the "Primary Site ID" field in the second site to make sure there can't be multiple entries in the second site per item in the primary. There are a few more things you may want to consider in doing that.
It would be a good idea to break permissions on that secondary site list and restrict who can create entries or at the least hide the "Primary Site ID" field from editing, so that no one can create an entry and use one of the "Primary Site ID" values that your Flow would need.
Another thing to be aware of with this Flow is to ensure you don't create circular triggers. If both of the Flow triggers include "When an item is Modified" or "When an item is create or modified" then they will go into an infinite loop by triggering the other each time the one is triggered.
For additional details, the MS Flow blog has a post which describes a similar task. Note that it is dated and has an inaccuracy regarding the "When an item is deleted" trigger which has since been added. Link below.