Merging two spreadsheets

[BrazzellMarketing]

I think I'm going to end up with two spreadsheets for customer tracking info. One spreadsheet will have about 8,000 lines. The other will have about 10,000 lines. The common element between the two (Key) is the customer numbers. How do I combine the data into one spreadsheet where information about one customer in the shorter spreadsheet will show up in the same customer number line in the long spreadsheet?