Custom Fields are added to forms by System - Fields
plugin.
Before doing template override I suggest checking out Field Groups feature. Each field group has its own fieldset which results in a separate tab in form view:

If this solution doesn't suit you, you can override form view layout:
administrator/components/com_content/views/article/tmpl/edit.php
In the override add fields-0
fieldset to ignored fieldsets array:
$this->ignore_fieldsets[] = 'fields-0';
And then render the fields manually:
<?php echo JHtml::_('bootstrap.addTab', 'myTab', 'fields', JText::_('JGLOBAL_FIELDS')); ?>
<div class="row-fluid form-horizontal-desktop">
<div class="span6">
<?php echo $this->form->renderField('field-1', 'com_fields'); ?>
</div>
<div class="span6">
<?php echo $this->form->renderField('field-2', 'com_fields'); ?>
</div>
</div>
<?php echo JHtml::_('bootstrap.endTab'); ?>
You can also combine both methods to avoid having to render each field by name and instead use fieldsets (field groups). E.g. create two field groups, assign fields to them. In the override exclude the groups:
$this->ignore_fieldsets[] = 'fields-1';
$this->ignore_fieldsets[] = 'fields-2';
And then render them:
<?php echo JHtml::_('bootstrap.addTab', 'myTab', 'fields', JText::_('JGLOBAL_FIELDS')); ?>
<div class="row-fluid form-horizontal-desktop">
<div class="span6">
<?php echo $this->form->renderFieldset('fields-1'); ?>
</div>
<div class="span6">
<?php echo $this->form->renderFieldset('fields-2'); ?>
</div>
</div>
<?php echo JHtml::_('bootstrap.endTab'); ?>