you can, where you see in central admin the content db you can add another content database! when creating a site collection you need to stop the first content db that you dont want it in and keep the other enabled... when you create the site collection it would be added to the active conetent database! than reenable the one you stopped and hay presto you have two site collections on seperate content databases ;)
more info here with diagrams:
http://blogs.technet.com/b/sharepointcomic/archive/2009/07/21/unique-content-database-per-sharepoint-site-collection.aspx
for pre existing content database that contains more than one site collection that you want to split follow this msdn tutorial on splitting content databases:
http://technet.microsoft.com/en-us/library/cc825327(v=office.12).aspx
Split a content database (move a site collection to a different database)
1.At a command prompt on the drive where SharePoint Products and Technologies is installed, change to the following directory:
%COMMONPROGRAMFILES%\Microsoft shared\Web server extensions\12\Bin
2.Type the following command, and then press ENTER: Stsadm -o enumsites -url > .xml
where url is the address of the Web application that contains the site
collection that you want to move, and path/file name is the name of
the XML file that you want to create with the site collection data.
For more information, see Enumsites: Stsadm operation (Windows
SharePoint Services).
The previous step creates an XML file that contains all of the site
collection URLs in the current database.
3.Open the XML file that you created in a text editing application. If there are any URLs for site collections that you do not want to move,
be sure to delete them from the file. The only URLs that should remain
in the XML file should be for the site collections that you want to
move.
Note: There is no need to change the site count or any of the other
site collection information in the file. Only the URLs are relevant to
this procedure.
4.Type the following command, and then press ENTER: Stsadm -o mergecontentdbs -url -sourcedatabasename
-destinationdatabasename -operation 3 -filename
where url is the address of the Web application that contains the site
collection that you want; sourcedatabasename is the name of the
database that you want to move the site collection from;
destinationdatabasename is the name of the database that you want to
move the site collection to; operation 3 is the “Read from file”
operation; and file name is the name of the file that contains the
site collection URLs that you want to move.
For more information, see Mergecontentdbs: Stsadm operation (Windows
SharePoint Services).
5.Restart IIS by typing the following command, and then pressing ENTER: iisreset /noforce