You can't combine two different dashboard sheet to one dashboard sheet. You should use different worksheets to achieve your task.
I am not sure this will help you or not but still giving you suggestion to club all the dashboards into one screen, you can use the dashboard fixed position by increasing the size (height of the dashboard), now from here on you can plot all the created sheets in to one dashboard. But make sure performance might impact according to the best practices of Tableau.
It would help us answer if you can provide some examples probably visuals hand drawn or workbooks itself.