You can create an aggregate key using 'Scripted Fields'. In Kibana, starting from 5.0, there is a UI to do this: https://www.elastic.co/guide/en/kibana/current/scripted-fields.html.
a) As long as you can distinguish for each document which excell-sheet it belongs to from one of the field-values in the document, in Kibana you will be able to tease out both excell sheet; when designing visualizations, adding a query to the filter bar, etc...
b) If your documents are in different indices, that not a problem. In Kibana, you will need to create an "Index Pattern": https://www.elastic.co/guide/en/kibana/current/tutorial-define-index.html. This index pattern can contain wildcards, so it can match multiple indices at the same time. e.g.
excell-* would catch index
excell-a and index
Regardless, when you will be running queries and make comparisons between the two, you will still need a field-value per document that establishes which excell sheet they belong to.
So, if you want to e.g. make 2 line-charts that show metirics per excell-sheet, you will need (a). If you also want to have a clean index layout, and run index-level stats, you could opt to split them up with (b) and use an index-pattern with wildcard to run queries over both.