I've been trying to find a way to make one of two fields valid and haven't come up with much. Is there a way to do this with custom field validators or is that just relevant to individual fields? The requirement is that one of two fields must be filled in, probably only one, but even if that's not possible, just having at least one filled in (and then there being a default in terms of which is used) would be acceptable, I suppose.

There is nothing out of the box that will do this for you as far as I am aware and although you think it might be possible custom validation I don't think it is as the a validator works on only one field at a time so updates to the item would only have the updated data for that single field and not the other field. Therefore I think your only option here might be to use the item:saving event to validate the data there as mentioned by Richard Seal below:

Item validator access already stored data instead of newly inputed data

You need to be careful with this though to make sure it doesn't fire all of the time. Maybe you could check the template before running this validation or something.

Another related thread on SSE is here:
Apply field validations across fields

I would recommend implementing a custom Item validation rule.

Note: This is different from a field validation, in that it validates the entire item instead of a single field.

1. Create custom validator

Create a new class that implements the Sitecore.Data.Validators.StandardValidator base class. In the Evaluate() method, get both fields and check to see if either of them has a value:

public class AtLeastOneFieldPopulatedValidator : StandardValidator

{

    public override string Name => "At Least One Field Populated";



    public AtLeastOneFieldPopulatedValidator() {}



    public UrlCharacterValidator(SerializationInfo info, StreamingContext context) : base(info, context) {}



    protected override ValidatorResult Evaluate()

    {

        Item item = base.GetItem();

        if (item == null)

        {

            return ValidatorResult.Valid;

        }

        if (!string.IsNullOrEmpty(item["Field 1"]) || !string.IsNullOrEmpty(item["Field 2"]))

        {

            return ValidatorResult.Valid;

        }

        return base.GetFailedResult(ValidatorResult.CriticalError);

    }



    protected override ValidatorResult GetMaxValidatorResult()

    {

        return base.GetFailedResult(ValidatorResult.Error);

    }

}

2. Create Validation Rule item

First, create a new Validation Rule (template /sitecore/templates/System/Validation/Validation Rule) item somewhere below this node:

/sitecore/system/Settings/Validation Rules/Item Rules

Important: Make sure you populate the Type field with your fully-qualified name of your custom validator's class (e.g. Custom.Services.AtLeastOneFieldPopulatedValidator, Custom.Services).

3. Associate your Validation Rule to your items

On the __Standard values of your item's template, show Standard Fields (View ribbon tab -> Standard fields) and add your Validation Rule to the appropriate validation fields. I recommend including it in all four.

(Optional) Extra credit: Add parameters

I've not tested this, but I'm sure there's a way to pass parameters using the Parameters field on the Validation Rule item. In these parameters, you could pass a list of the fields to check so your validator can be reused by creating multiple Validation Rule items in Sitecore.