i dont believe that requip xl actually works constantly for 24 hours. most drugs seem to be in the system in a bell curve. the second dose might be to fill in the downside of the bell curve to make a plateau. according to the FDA the median time to peak concentration is 6 to 10 hours,ie usually about 8 hours.
it might look a bit like this where v is 8mg and is 4mg.
eating a fatty meal also increased the AUC (area under the curve) and the c max.http://www.drugs.com/pro/requip-xl.html