When you connect an account, Reckon asks how many card members are on it. If two people have cards on one account, answer "Two". That answer is what lets Reckon notice if the bank sends both people's charges merged together.
In Settings › "Accounts and connections", each card account shows its card members, and on the Month page the "What came in" panel marks each account with its card members' initials. A charge the bank did not attribute to a card member is counted as unresolved.
"Unresolved" means the bank did not say which card made that charge. Reckon does not guess. Unresolved charges are counted on the page and highlighted on every tab of the spreadsheet export; they are never quietly merged into one person's charges.
To resolve them, bring in the card issuer's own CSV export: "+ Bring in a statement" (in Settings › "Accounts and connections", or on the Month page's "What came in") opens the Bring in sheet; choose "Statement file". Reckon recognises the issuer itself, and that export names the card member on every row.
Charges on accounts with a single holder are never counted as unresolved — there is no card member to attribute. They are still in the register and in every export.