Most airlines have a policy for large passengers' purchasing a second seat that might not extend to anybody's purchasing a second seat for convenience. With this policy the two purchased seats should always be together or the gate agent has the right to re-assign people as needed (seat assignments are never guaranteed if I read several other posts on this forum correctly)

I don't think the airline can bump just the second seat without following all the bump procedures and at the option of the passenger bump the first seat as well and pay two sets of compensation.

In my opinion (which does go far enough to satisfy many road warriors) (1) A large person having purchased two seats is entitled to raise one armrest, and (2) any person aggrieved by a large person sitting next to him is entitled to be reseated anywhere else in the plane. Under these conditions, if a large person's 2 seats are not adjacent, the problem will always resolve itself.

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