If a supporter is sponsoring more than one sponsorship, you would enter a row with the supporter information including the information for the first sponsorship - sponsorship_id and shares. Then, copy the complete row with all the supporter information and paste it in the next row changing the sponsorship_id to the permalink of the second sponsorship. Update the shares to the number of shares they are sponsoring for the second sponsorship.
REACH will recognize that this is the same supporter with a second sponsorship. Instead of creating a new supporter record, REACH will automatically connect the supporter to the second sponsorship.
You can repeat this process on as many rows as necessary to include all sponsorships sponsored by the supporter.