Yes. An EVP Systems Account Code is just used for billing purposes, so you can use the same Account Code for any number of EVP Office and EVP Everywhere installations. Their usage will just appear on the same invoice.
EVP Everywhere is a webservice that allows other software to easily retrieve the same estate-tax valuations that EstateVal produces, in a machine-readable format. If you want to integrate EVP Systems data into your own applications, EVP Everywhere is how you do it. You can learn more at the EVP Everywhere website.