A couple months ago I stopped being able to DL twice a week (had been doing reg. Novice with DL/PC on A/B). I ended up doing heavy PC and heavy DL once a week (each on different days), and used the remaining day for light PC practice for form. This was a good idea and the spacing helped me continue progress on both exercises, as well as the squat, for maybe a month or so longer than would have been the case. The "extra" day not used for heavy PC/DL gives a lot of flexibility and could easily feature back extensions or fast DL, changing according to goals and gas in the tank.

Here's a link to log if above isn't clear: http://startingstrength.com/resource...ad.php?t=18553