bootstrap 4 create modal footer with multiple rows

Yes for sure you can place two modal-footer elements.

<div class="modal fade" id="exampleModalLong" tabindex="-1" role="dialog" aria-labelledby="exampleModalLongTitle" aria-hidden="true">
          <div class="modal-dialog" role="document">
            <div class="modal-content">
              <div class="modal-header">
                    <h5 class="modal-title" id="exampleModalLongTitle">Modal title</h5>
                <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                  <span aria-hidden="true">&times;</span>
                </button>
              </div><div class="modal-body">
                ...
              </div>
                <div class="modal-footer">
                    <button type="button col-lg-12" class="btn btn-secondary" data-dismiss="modal">Close</button>
                </div>
                <div class="modal-footer">
                    <button type="button col-lg-12" class="btn btn-primary">Save changes</button>
                </div>
            </div>
          </div>
        </div>

Then in your css add this

#exampleModalLong .modal-footer .btn{
    width:100%;
}

Here you have a jsfillde link

A more "natural" way to do it would be to add

.modal-footer {
  display: block;
}

It's the default display: flex that makes the buttons share the same row.