I would consider the 3 field option for this. Having some of your data in the other field becomes a real pain.
Field 1 is the select_one and required
Field 2 is a text box with a relevant of Field1 = Other and required
Field 3 is a calculate type with a if statement to grab Field 1 or Field 2.
Set the bind esri to null for both Field 1 and Field 2 to null so it never even goes to the service.
One downside is domains. One option for that is to also have Field 3 a select_one and use the esri visable column to hide it from the user.
Just an idea.