Hi Arkesh, 

When there are multiple attestation respondents present, they all need to fill in the assessment. 

But, when you only have one attestation respondent, it does not mean that only that person can perform the assessment. If you have an owning group assigned to the control, anyone within the owning group can perform the assessment. 

I would not recommend you to customize this as it brings some hassle whenever you upgrade to the newest version. But you could probably accomplish it with a business rule. 

Hope this helped you a bit!

Good day!

Lana