Roof racks for tandems are expensive. I guess you found that out. In addition a Wal-Mart (Do they sell tandems?) is likely to be so heavy that just getting it on the roof rack would be enough of a workout for the day. This might not be such a great present for a couple of reasons.
1. Riding a tandem is a step harder than riding a single bike.
2. Cheap tandems don't get much use and if they do, they break.
3. It's hard to say whether two people will work out ok on a tandem. There is a lot of verbal and non-verbal communication.
If your really convinced look for a used better quality tandem. Tandems from the 80's can be had for around $300 and several are in very good condition.
Frank