Sure, you could do it if you can stomach flying on the cost of a "full fare" ticket. Not me.

Southwest's -discounted, nonrefundable advance fare- tickets are fully refundable in the form of vouchers, with no penalty. That's the trick. You can do the above with 3 $50 one-way tickets, and get a $100 voucher with the two that you do not use.

Try that with United and you'll have to pay an obscene amount for that refundable fare. If you do it with advance tickets, you eat $75 on each one. It's just not worth it.

If you REALLY want to do this on the cheap (tho I haven't tried it yet), reserve those seats using Rapid Rewards vouchers. You can get bumped with no upfront cost to you, but check the blackout dates. This one's a little riskier, since they may be able to involuntarily bump you with little or no compensation.