I understand I am limited to 5 Mailbox Databases in Exchange 2010 Standard. If I own a second Exchange standard license, can I setup a second Mailbox server and have more mailbox databases in the same en

Yes.  But each server can only have a maximum of 5 databases mounted at a time.  If you are standing up a second server to add more databases, you may want to consider the enterprise license so each server will be able to mount all the databases in the event of a failure (network, software, or hardware).

If you are not going to use Second server for resiliency purpose (and not configuring the DAG) and going to use it as a stand alone server then Yes, else what Hinte said...

Hi,

I recommend this blog to understand the mailbox database limit in Exchange 2010 Standard.

http://blogs.technet.com/b/ilvancri/archive/2010/04/22/exchange-2010-standard-edition-limited-to-5-databases.aspx

Best Regards.