Sounds like the OP is not talking about certs (ecerts/TCVA) but actual tickets in their name with value left on them. These work completely differently then certs. However, the end result is correct - since the old tickets remain with their value, but without an itinerary for them.

The only way to do what you want is to do what GregMM suggests above. Book two one ways. Note that depending on route, two one-way fares may be more (and potentially significantly more) than a round trip.

As for the two-people suggestion, assuming this is canceled ticket credit, each credit can only be used for the passenger that was originally ticketed. Canceled ticket credit essentially is just doing a change, without specifying new flights at the time of canceling the old ones. When you know what you want, you are using the same ticket for the same passenger on the new route. You'll pay the change fee (in new $) and then have the full value of the old ticket to use. If the new ticket is less, you'll get a new TCVA to use on future travel (this can be used for anyone).

One further note - since the new travel is on the old (re-issued) ticket, you will not be able to take advantage of canceling within 24 hours for a full refund. A new change 5 minutes after purchasing the new flights will result in another change fee.