Although DocuSign allows you to validate certain fields, even with custom validation it does not allow you to validate across multiple fields. An example of when this is helpful is on some forms a certain group of boxes may be required to add up to a certain amount (such as percentages that must add up to 100). You can use tooltips and alert messages to remind the signer, but these will not stop them from completing the form if they made a mistake. We will use percentages that must add to 100 as an example.
First you want to create two formulas, one that adds up the percentages and another that checks to see if that total is equal to 100. The second formula will evaluate to 0 when the total is not 100, and 1 when the total is 100.
Formula1: [X] + [Y] + [Z] (X, Y and Z being the percentage fields)
Formula2: [Formula1] = 100
Now the trick is to create a field that will always be invalid no matter what value is entered and make that field appear when the second formula equals 0 (meaning that the total does not add up to exactly 100).
In order to make the field impossible to fill out, you must create a duplicate field, giving it the same data label as the original and making it invisible (Read Only and white text). You can then give the two fields mutually exclusive custom validations. For example, you can make the first accept only digits 0-4 and the second digits 5-9.
Regex pattern: ^(0-4)
Regex Pattern: ^(5-9)
You can also customize the error message to remind the signer of the problem they must fix.
Since the fields have the same data label, any value typed in one will automatically appear in the other, making it impossible to satisfy both validations. This forces the user to fix the issue in order to make the warning and field to disappear, allowing them to complete the form.