Are you really sure one person can check in twice?

At Gatwick for example, you have to self scan BC to get into security. And only have one chance to do this. You simply can't scan two. The personnel aren't gonna scan two for one person. If you present an unscanned BC at gate it's gonna get flagged as the passenger hasn't gone though security. So if the BC isn't scanned, the passenger has not shown up, and seat could be resold.

If you managed to check-in twice, the paperwork won't match the physical head count. and cause a right problem.

You try and mess with the above and get a heavy security response.

Luggage is also a personal allowance, not a ticket allowance.