hi im sure this is a basic question for you guys, but i need some help validating two fields so for example say one field has an input in i dont need to validate the other vice versa,

this is my HTML mark up and im focusing on the 2 fields from the last on which are "Where did you see the vacancy advertised?" and "Who recommended you"

<div id="ValidationSummary">
           <%= Html.ValidationSummary("Please fix the following problems") %>
          </div>
           <% using (Html.BeginForm("job_application", "vacancies", FormMethod.Post, new { enctype = "multipart/form-data" }))
                  { %>
        <div class="blue-border-form-details">
            <h1>Personal Details</h1>
                     
             <fieldset>
                    <legend><label>* Title:</label><%=Html.DropDownList("Title", Model.TitleCollection())%></legend>
                    
                    <legend><label>* First name:</label><%=Html.TextBox("FirstName", Model.FirstName, new { maxlength = 50, size = "30", autocomplete = "off" })%></legend>
                    <legend><label>* Surname:</label><%=Html.TextBox("SurName", Model.SurName, new { maxlength = 50, size = "30", autocomplete = "off" })%></legend>
                    <legend><label>* Email:</label><%=Html.TextBox("EmailAddress", Model.EmailAddress, new { maxlength = 50, size = "30", autocomplete = "off" })%></legend>
                    <legend><label>* Mobile:</label><%=Html.TextBox("Mobilenumber", Model.Mobilenumber, new { maxlength = 11, size = "30", @class = "number", autocomplete = "off" })%></legend>
                    <legend><label>* Work Eligibility:</label> <%= Html.RadioButton("EligibleToWork", Model.EligibleToWork,true)%> I confirm that I am eligible to live and work in the UK</legend>
                    <legend><label>&nbsp;</label> <%= Html.RadioButton("NotEligibleToWork", Model.NotEligibleToWork,false)%> I am not currently eligible to live or work in the UK</legend>
                    <h1>Application Details</h1>

                    <div class="white-space"></div>
                    <legend><label>* Title:</label><%=Html.DropDownList("JobTitles", Model.VacanciesCollection())%></legend>
                    <legend><label>* Location:</label><%=Html.DropDownList("Locations", Model.LocationsCollection())%></legend>
                    <legend><label>* Attach CV:</label>How to upload your CV. Just click on the browse button below, find your CV in your<br /> files and add it. We can accept PDF or Word documents no larger than 500kb.
                       
                    <br /></legend>
                    <legend><label>&nbsp;</label>Alternatively copy and paste your CV into the covering letter area.</legend>
                    <legend><label>&nbsp;</label><%= Html.TextBoxFor(m=> m.CV, new { type= "file" }) %><br /></legend>
                    <legend><label>Cover letter:</label><%= Html.TextArea("CoverLetter", Model.CoverLetter, new { maxlength = 2000, size = "300", @class = "text-area-enquiries", autocomplete = "off" })%></legend>
                    <legend><label class="large-label">Where did you see<br /> the vacancy Advertised?</label><%= Html.TextBox("VacancyAdvertised", Model.VacancyAdvertised, new { maxlength = 30, size = "30", autocomplete = "off" })%> </legend>
                    <legend><label class="large-label">Who recommended you?</label><%= Html.TextBox("RecommendedYou", Model.RecommendedYou, new { maxlength = 30, size = "30", autocomplete = "off" })%></legend>
                    <legend><label>&nbsp;</label><%= Html.CheckBox("CopyofApplication",Model.CopyofApplication)%> I wish to receive a copy of this application.</legend>
            </fieldset>

        </div>
        <div class="white-space"></div>
        <div>
            <span class="margin-left"><a href="/vacancies/current-vacancies"><img src="/img/buttons/Back-to-Job-Description-Button.png" alt="Click to go back to Job Description" class="margin-left" title="Back to current vacancies"/></a></span>
            <span><input type="submit" name="Submit" id="Submit" class="offset6 apply-now-button submit" value="Apply Now" title="Apply now | Send CV & application" runat="server" /></span>
        </div>
        <%= Html.ClientSideValidation("", typeof(BpdJobsModal.JobApplication))%>
                <% } %>
      </div>


This is the code in the modal again how can i combinded both of these in to one, make sure one of the two fields have a value in

 [Required(ErrorMessage = " * Required")]
        [RegularExpression(".{2,}", ErrorMessage = "Invalid Required Field")]
        public string VacancyAdvertised { get; set; }

        [Required(ErrorMessage = " * Required")]
        [RegularExpression(".{2,}", ErrorMessage = "Invalid Required Field")]
        public string RecommendedYou { get; set; }

Any help will be highly appreciated.

Thank you