Central Admin doesn’t allow you to choose in which Content Database to create a new Site Collection. With PowerShell its easy:
New-SPSite ` –ContentDatabase "WSS_Content_SecondDB" ` -Url "http://portal.contoso.com/sites/MyNewSiteCollection" ` -Template "STS#0" -Name "SitecCollectionTest" ` –Description "A Site Collection created in a separate Content Database" ` -OwnerAlias "CONTOSO\Gerben" ` –OwnerEmail "gerben@contoso.com" `