Try adding the following script to the from. It will auto-populate the People Picker column based on the lookup column.
<script language="javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script language="javascript">
$(document).ready(function () {
// the lookup field in the new form
var IssueIDField= $("select[title='Dept']");
IssueIDField.change(function () {
Populate();
});
});
var ListItem;
function Populate() {
var IssID = $("select[title='Dept']").val();
var clientContext = new SP.ClientContext.get_current();
var IssueList = clientContext.get_web().get_lists().getByTitle('cl01');
ListItem = IssueList.getItemById(IssID);
clientContext.load(ListItem);
clientContext.executeQueryAsync(Function.createDelegate(this,this.Succed),Function.createDelegate(this,this.Failed));
}
function Succed(sender, args) {
var context = SP.ClientContext.get_current();
var web = context.get_web();
var user = web.ensureUser(ListItem.get_item("HOD").get_lookupValue());
// Get People Field
var form = $("table[class='ms-formtable']");
var userField = form.find("input[id$='HOD_17d85e7c-80fe-4131-8b50-c5d456fdd449_$ClientPeoplePicker_EditorInput']").get(0);
var peoplepicker = SPClientPeoplePicker.PickerObjectFromSubElement(userField);
// clear people Picker
while (peoplepicker.TotalUserCount > 0) {
peoplepicker.DeleteProcessedUser();
}
context.load(user);
context.executeQueryAsync(function () {
//execute successfully
//this is a user field
// Set the People field
var loginName = user.get_loginName();
peoplepicker.AddUserKeys(loginName);
}, function (sender, args) {
//execute fail, so it isn't user field, it may be a group
var collGroup = context.get_web().get_siteGroups();
//get the people group via Id
var oGroup = collGroup.getByName(ListItem.get_item("HOD").get_lookupValue());
context.load(oGroup);
context.executeQueryAsync(
//execute successfully
//this is a group field
function () {
// Set the People field
var loginName = oGroup.get_loginName();
peoplepicker.AddUserKeys(loginName);
},
//execute fail, it is neither user nor group
function (sender, args) {
contoso.log('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
})
});
}
function Failed(sender, args) {
console.log('Error. ' + args.get_message() + '\n' + args.get_stackTrace());
}
</script>
Change the 'cl01'
to your 'List1'. And change 'HOD_17d85e7c-80fe-4131-8b50-c5d456fdd449_$ClientPeoplePicker_EditorInput'
to the id of your HOD column in 'List2'.
Use F12 to check the id of the element.

Refer to Auto-populate list form fields based on lookup selection in SharePoint.