Related to my comment above, this could be achieved by bypassing the convenience method and wiring up the filters separately on your app startup:

    SwaggerSpecConfig.Customize(c =>
        {
            var actionCommentsDoc = new XPathDocument(GetActionCommentsPath());
            c.OperationFilter(new ApplyActionXmlComments(actionCommentsDoc));

            var typeCommentsDoc = new XPathDocument(GetTypeCommentsPath());
            c.ModelFilter(new ApplyTypeXmlComments(typeCommentsDoc));
        });

The existing config method is for convenience in the simplest case (i.e. a single Xml file) and I think the introduction of a second parameter may be a little confusing - it's also a breaking change to the "public" contract of the config API.