It is easier to do it all in one database and have access to all the tables, but there may be reasons one would want to use 2 different databases, personally, I prefer it that way. Imagine your database goes down, and ALL information is lost, or you want to use the contact manager database for another database, or for security reasons want to lock down the invoice one, or the contact one (or preferably both) for privacy reasons. As for an education on how to do it, that is best left to the experts to teach you, and I am sure you are like the rest of us, trial and error, and by example are good teachers. Download or use some template databases, of both types and see how you like modifying them and working with them, only once you get your purpose straight will you be able to get the direction down as well of how to plan it.
Hope that helps,