I have two separate companies in the same building(tower), each has a complete cisco LAN infrastructure, what is the best solution to make it as a one LAN providing security for both sites?

What is your purpose for connecting both sites? Is it to share an Internet connection? Share files and emails? Do you intend on having a centralized network administration? Do you want to delegate this authority to individuals in each company? How are the companies related? Are they part of a parent coporation?