Conditional statement make checkbox yes or no, what am I doing wrong.

Conditional statement make checkbox yes or no, what am I doing wrong.

So I have two fields, one where you select a employee(Managing_Therapists) in a lookup field and the other if a signature field (Managing_Signature) is signed. 

I want to Mark a Checkbox Yes if both Conditions are met, and no if one or both fields are empty. Every time I remove the signature field (Managing_Signature) on edit the Checkbox stays YES.

The following workflow is executed on edit of the form. 

if(input.Managing_Therapists == null || input.Managing_Signature == null)
{
input.ManagingTherapistCheckbox = "NO";
}
else if(input.Managing_Therapists != null && input.Managing_Signature != null)
{
input.ManagingTherapistCheckbox = "Yes";
}

Any help is appreciated.