I asked the same question myself ,I was informed that it was not nessary too. I myself did not split them apart, ( the rebuild kit does not have any o-ring for the inside ports of the split section) and they seem to be fine I'll not know for sure till I try them out tho,
2003Z & Cruez are Pro's so I'd listen to thier advise.
I did totally rebuild the calipers after coating, and assembling the rear calipers would make the Pope curse