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 "https://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" `