The error here is “tastes great” as this can be only used when talking about a single unit, while we are currently talking about two units (peanut butter and jelly).
The correct way to phrase this is: “Peanut butter and jelly are two products that taste great by themselves and taste great together, too.”
The sentence contains subject-verb disagreement, which occurs when the subject is plural and the verb is singular, or vice versa. In this case, the plural subject <em>Peanut butter and jelly</em> does not match with the singular verb <em>tastes (great)</em>. In fact, the previous plural verb <em>taste </em>great actually corresponds with the plural subject.
