I use W&R Fable and R&S English 5. It has not been a problem at all to do both. W&R lessons are typically done in about 3 days by my son (4th grade and writing phobic). I use WWE3 narration days on the other 2 days of the week (I skip the WWE dictation, since W&R has dictation). I don't plan to use every R&S English writing lesson, but the lesson we did recently was writing about a personal experience, with the example being a barn fire. It just so happens that we had a garage fire last year, so I went ahead and had my son write about that, and we came up with various questions to ask and put them on the board. He wrote from that list of questions, picking and choosing. We did not do W&R that day, nor did we do any writing in WWE that day (only one of the two narration days in WWE involves writing).
As far as the grammar lessons go, he still does those each day, including the written exercises. The more my son writes, the easier and more automatic it gets. I assign about half the exercises.