I don’t have a 100% clear idea of what is happening. I guess you are making the sleeves at the same time as you are working the rest somehow already? You have 49 stitches at the beginning of where you give the directions. Anyway when it says at the same time, it means that you need to think ahead and pay attention and begin to do this other part when your sleeve (or is it sleeves) are 18 inches long. Keep doing the first part (rows 10-32) and watch the sleeve length. When it reaches 18 inches and you have just completed a wrong side row you will need to begin to the shape the body as well as keep doing what you were (although it won’t be as wide anymore). When you measure the sleeve and it is 18 inches, if you have just done a right side row, do one more row, or you could stop a bit short of 18 if that seems better, but as close to 18 inches as you can get and end on a wrong side row is where you need to be.
You now shape the body. You will be at the beginning of a right side row. Bind off 18 stitches and knit 31 (this accounts for the 49 you had at the beginning), you don’t add other needles or anything you just work at the beginning of the row and get rid of 18 of the 49 you had. You can keep following the directions for rows 10-32 where ever you were in that sequence because the changes you make there are at the other end of the row from where you did the bind off of 18. (It looks like row 10 above is a wrong side row) You keep making the 5 stitches of garter at the front.
It says to keep working until you are 11 inches from the sleeve bind off row. Maybe there is something after row 32 that you haven’t given. I don’t see how this part fits in from what you have said, but I think you didn’t tell us everything.
After you have finished row 32, of the above part you gave, you go on to whatever it gives next and then keep going until you are 11 inches past where you bound off the sleeves. Then knit 8 rows of garter edging and bind off. If there is not anything else given after row 32, I could be all wrong about this.
At the same time means that the two tasks will overlap. You will still be doing the first part and then at a certain point you will begin to do the next thing, but keep doing the first part as well.