question

You'll have to write some custom code for this. I would generate one value as normal, then generate another value until the sum of both doesn't exceed 100. One value is assigned to the current item, the other one is stored in a label on the source, so it can be assigned to the next item. If no stored value exists, a new value pair is generated.

In the example model I use an empirical distribution to generate the random values, so the code is less cluttered and hopefully easier to understand.

custom-percentage-label.fsm

/**Custom Code*/
Object current = ownerobject(c);
Object item = param(1);
int rownumber = param(2); //row number of the schedule/sequence table

// Make sure that source labels exist
current.labels.assert("StoredValues", []).value;
current.labels.assert("Index", 0).value;

if(current.StoredValues.length == 0)
{
    // No more stored values -> Create new ones and increment index
    double firstValue = Empirical("TestDistribution").get(getstream(current));
    double secondValue = Empirical("TestDistribution").get(getstream(current));
    while(firstValue + secondValue > 100)
    {
        // Keep rerolling until sum is not larger than 100
        secondValue = Empirical("TestDistribution").get(getstream(current));
    }
    // Update source labels
    current.StoredValues = [firstValue, secondValue];
    current.Index += 1;
}

// Assign labels to item
item.Value = current.StoredValues.shift();
item.PairID = current.Index;
item.color = Color.byNumber(item.PairID);