I have worked for 2 different companies in 2007. The second company I joined was in December only. So how do I have to file tax return. Can federal and state tax be filed together?

Yes, you merely combine the incomes, many people may work for 4 or 5 companies, For example, I worke actually for 3 companies, and did selp employment with two separate 1099's, for w2 all you do is combine the incomes