How to send values from two different forms to an insert?

I need to save the ID of the event I want to register the presence of a certain student in the Bank, but I also need to save some other data, so I created a <?php if(!isset($_POST['evento'])):?> But I need to send the value of both my form that only "looks for" where it contains the event / idevento, and of the values that I inserted for the presence of the student. Here is how the form code is currently. What would I need to spend to send all these values all at once?

<?php if(!isset($_POST['evento'])):?>
                    <form method="POST" >
                    <input type="hidden" name="evento" value="QQCOISA" hidden/>              
                    <h3>Nome do Evento</h3>
                    <select name="idevento">
                        <?php
                        include "conectar.php";
                        $sql = "SELECT * FROM evento";
                        $result = $conn->query($sql);
                        if ($result->num_rows > 0) {
                            while($row = $result->fetch_assoc()) {
                                echo "<option value='".$row["idevento"]."'>".$row["noeve"]."</option>";
                                $idkappa = $row["idevento"];
                            }
                        } else {
                            echo "Sem eventos ativos";
                        }
                        $conn->close();
                        ?>
                    </select>
                    <input type="submit" value="Selecionar Evento"/>
                    <?php else: ?>
                     <form method="POST" action="input.php">
                    <input type="hidden" name="pagina" value="inserirReg" hidden/>
                    <input type="hidden" name="idevento" hidden/>          
                    <h3>Dia Evento</h3>
                    <select name="seqDia"> 
                        <?php
                        include "conectar.php";
                        $sql = "SELECT * FROM diaevento WHERE idevento=".$_POST["idevento"];
                        $result = $conn->query($sql);
                        if ($result->num_rows > 0) {
                            while($row = $result->fetch_assoc()) {
                                echo "<option value='".$row["idevento"]."'>".$row["seqDia"]."</option>";
                            }
                        } else {
                            echo "Sem eventos ativos";
                        }
                        $conn->close();
                        ?>
                    </select>

EDIT: I did not see any need to put the end of code <?php endif; ?> , because there are only a few selects like Event Day.