I can't address the code issue, however if there are two vents to a crawl there is the possibility that they can become occluded if flooding occurs (not likely but...), hence the high/low requirement. There is a requirement for 1 square inch per 1,000 BTU venting (in this case 100 sq. in. per unit) with one vent high and one low (12" from ceiling/floor, as you said). The rationale is that separate vents prevent occlusion. This is the way I understand it.

Note also the type of vents may have different restiction qualities. Wood slat vents have about 25% usable vent area; metal screen type vents 75% usuable vent area. That's the rule of thumb I use.