Answer:

In the "Shell and Tube Heat Exchanger Properties" dialog box, under the tab "Geometry", check the field "Stacked" as shown, also make sure the inlet outlet options are correctly selected as shown on the right in the image below:

Model the rest of the project as if you are designing a single exchanger, selecting the "Stacked" option would automatically create 2 identical exchangers stacked on top of each other as shown below:

In case the saddles do not appear for the bottom or top exchanger, open both saddles one at a time and select option SB  and click OK and repeat the same and select option ST.