If you put the same email address twice, you'll never know which of the two people sharing the inbox actually signed it.
There are ways around that:
add extra security
send a document to person 1, add their mobile number
when person 1 signs, send the signed copy of the document to person 2, using their mobile number
at the end of the process, the signed document will have 2 certificates - each showing the same email address of the signer but different mobile numbers
use email aliases to speed up the process
if the address allows aliases (as does gmail.com), create two contacts:
person 1 will get joebloggs+1@gmail.com
person 2 will get joebloggs+2@gmail.com
send the document to both people using person 1 and person 2 email addresses, making sure each has a separate mobile number added as extra security
at the end of the process, the signed document will have a single certificate - showing email aliases and separate mobile numbers.