I usually save the text I want to reuse as a <ph> element in a DITA file that I store in a folder titled 'reuse'. You can use a <p> element if you prefer, I just find <ph> more flexible in where I can insert it later. Make sure the element has an ID defined. To do this select the element, then from the Structure menu > Assign ID to element. Save the file and keep it open.
Now, in the file you want the paragraph to appear:
Insert > Conref
In the file settings dropdown choose the reusable file with the text you want. All the elements in that file that have IDs assigned will appear in the list below. Choose the element you need. Click Insert.
You can repeat this process in the other file you want the text to appear in, pulling from the same reusable file.
Note that the reusable file must be open in order to show up in the drop down list.