Regardless of the printed restrictions, I've been able to combine vouchers from different "bumps" in the past. It all depends on the agent you find at the CTO. In fact, last time I was bumped on UA (less than a year ago), I was specifically told by the gate agent to disregard these particular restrictions printed on the voucher.

On the other hand, you may find a few agents who believe they need to enforce this policy... I'm not quite sure why it is printed that way when most ticket agents choose to disregard it.