One thing that you’ll notice if you export your customers out of Magento is that it combines the street1 and street2 columns into one for the shipping and billing addresses. The columns are called ‘billing_street_full’ and ‘shipping_street_full’.
If you then want to import addresses, this would seem like a hassle. The good news is that you don’t have to use that column for importing. Magento will gladly accept 2 columns for the street address if you just name them correctly. So, remove the ‘…_street_full’ columns, and add ‘billing_street1’ and ‘billing_street2’ and the same for shipping and you’ll be good to go.