Is HP Z400 SLI Support?

Solution
It’s not worth it, Sli has poor support in games these days. If you are using the copy of Windows that came from HP a motherboard change will invalidate your licence requiring you to buy a new Windows license.
You really need the motherboard model, HP Z400 appears to be a workstation range under that name.

It’s highly unlikely that a motherboard from such a workstation will support sli.

Also sli support is dying, personally I’d say sell the 770’s and get a single stronger gpu and avoid the issues and lack of support that comes with sli.