Shape Armhole and Front Slope: Bind off 4 sts at beg and dec 1 st at end of next row-25 (27, 29) sts. Work 1 row even. Dec 1 st at armhole edge on next 4 rows AND AT THE SAME TIME dec 1 st at neck edge every 4th row from previous dec 8 (9, 10) more times-13 (14, 15) sts. Work even until armhole measures same as back to shoulder, end P row."
On the first row, you'll bind off 4 st at the armhole edge and dec one st at the end of that row, the neck.
Work one row without dec.
Since the rate of decrease is different at the arm edge and the neck edge, they say 'at the same time' because you are working the dec simultaneously, but on different rows.
The best thing to do to keep track is to make a list of rows and mark which ones you'll decrease on the armhole side and which rows you'll decrease on the neck edge
1 dec arm
2 dec arm
3 dec arm
4 dec arm dec neck
8 dec neck
12 dec neck
Etc. until you have as many dec done as you need for your size.