See my comment hope that will help you, if not here is my explanation:
It depends upon what kind of form is this and why you need to do that, lets say you want the person's email address or phone no. any one from both. (AS YOU SEE HERE)
Then the situation may be diffrent. Is there anything we can put in dropdown? If Yes you can follow the example as in comment.
If I got this problem I probably put a label saying Fill atleast one feild to complete registration. And I put a completed tick mark illustration in order to inform user that you filled correct information. Like this suppose he filled in the first field the tick appears and he will know that this column of feild is completed let's move ahead.
This way is also possible to do. Enjoy