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" `