It will look like this, but I don't think that the position: fixed
requirement to make them stay as visible as the modals will be worth it. You should probably reconsider this idea.
$('#abtn').click(function(){
$('.modal').modal('hide');
$('#aaa').modal('show');
});
$('#bbtn').click(function(){
$('.modal').modal('hide');
$('#bbb').modal('show');
});
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>
<button class="btn btn-primary" id="abtn" style="position: fixed; z-index: 2000;">AAA</button>
<button class="btn btn-primary" id="bbtn" style="position: fixed; left: 4rem; z-index: 2000;">BBBB</button>
<div class="modal fade" id="aaa" tabindex="-1" role="dialog" aria-labelledby="aaaLabel" aria-hidden="true">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title" id="aaaLabel">AAAAA</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body">
AAAAAAAA
</div>
</div>
</div>
</div>
<div class="modal fade" id="bbb" tabindex="-1" role="dialog" aria-labelledby="bbbLabel" aria-hidden="true">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title" id="bbbLabel">BBBBB</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body">
BBBBBBB
</div>
</div>
</div>
</div>