I already have the 5-pound weights. Can I wear two ankle weights on one leg (i.e.: one 5-pound and one 10-pound leg weight? Purchasing two 10-pound weights would be less expensive than two 20-pound weights.

There are two possible problems with using two ankle weights on one leg. You might find them uncomfortable; or they might be harder to keep steady than a single ankle weight. Since you already have weights, why don't you try two on one leg to see how they feel. If they're comfortable and stable, then it's fine to use them that way.