There are lots of blogs available. You can refer How to Install OpenERP-7 on Ubuntu.

If you want to run multiple OpenERP servers like v6.0, v6.1 and v7.0 then refer How to Run Multiple OpenERP Server.

Hello,

You can not only install the 2 openerp server like.... 6,1 and v7 but you can install lots of openerp-server as per your requirement. Also you can start the openerp7 more then one at your system. Steps: Create a different user for each openerp server either v7 or v6.1. Create same super user on postgres. Alter the role on template database.

Start the openerp server with following command and assigned the different port for each server. --xmlrpc-port=7777 --netrpc-port=8888 (This is the server parameter here you can defined on which port you have to run particular openerp services)

Now you can run the openerp with assigned port.

Thank you!