RE: Embed Dashboard inside another Dashboard using iframe
I did have similar issues when i tried to display one CRM record within another using IFrame, its due the way the modern CRM pages are rendered, it does not behave well when we render a crm page within an Iframe in a CRM page.
You could try the following and see if that helps at all
What if you create an HTML web resource, have the content as shown below
<!DOCTYPE html>
<html>
<body>
<h2>Dashbaords</h2>
<table style='width:100%'>
<tr>
<td style='width:50%'>
<iframe src="yourcrmorg.crm0.dynamics.com/.../dashboard.aspx{GUID}&dashboardType=1030&pagemode=iframe"></iframe>
</td>
<td style='width:50%'>
<iframe src="yourcrmorg.crm0.dynamics.com/.../dashboard.aspx{GUID}&dashboardType=1030&pagemode=iframe"></iframe>
</td>
</tr>
</table>
</body>
</html>
And then you can have this web resoruce as Site Map tile link,