Here we go.... assuming constant sum is "Q1" and second question is "Q1dk" and is a multiple response question (checkbox) with only one answer option ("Don't know anything")...
In Q1, you need to uncheck the "Require response to question" and in Q1dk you need to "Allow respondents to check any number of check boxes"
In the "Custom javascript verification" area for Q1.... paste in the following...
if(((document.mainform["Q1_1"].value==0)||(document.mainform["Q1_2"].value==0))&&(document.mainform["Q1dk_1"].checked==false))
{
strErrorMessage = "Please enter an answer OR select Don't know anything";
}
if(((document.mainform["Q1_1"].value>0)||(document.mainform["Q1_2"].value>0))&&(document.mainform["Q1dk_1"].checked==true))
{
strErrorMessage = "You cannot enter an answer AND select Don't know anything";
}
if(((parseInt(document.mainform["Q1_1"].value))+(parseInt(document.mainform["Q1_2"].value))!=100)&&(document.mainform["Q1dk_1"].checked==false))
{
strErrorMessage = "Your answers do not sum to 100";
}
There may be a more elegant way to do it... but it should do what you need it to. Let me know if this doesn't work
Cheers
Russell