You need the 7960 rackmount chassis -- that has a second CPU option.  The tower systems you've listed do not.

As the number of CPU cores per processor has increased, there's likely less market for dual CPU desktop workstations.

I assume its ok to use it as a normal PC?

Although I cannot see 2nd processor option in 7960.

https://www.dell.com/en-uk/shop/workstations/precision-7960-rack-workstation/spd/precision-r7960-workstation/xctopr7960emea_vp?configurationid=2a341d18-7192-4bf4-9532-c975f09c278c

They're definitely available in dual CPU configurations, at least in the US.  That said, rackmount servers are designed to be installed in server rooms, so be sure you can live with the noise level -- they're not designed to be used in office settings are they tend to run very loud relative to what a desktop tower does.

https://www.dell.com/en-us/shop/desktop-computers/precision-7960-rack-workstation/spd/precision-r7960-workstation/xctopr7960usai_vp