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.
I expect the motherboard doesn’t support sli. Usually only high end motherboards support this feature and OEM’s don’t normally use high end motherboards.
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.