If you are doing the back to back parayanam by yourself, you could start the first chapter right after you complete the 13th chapter. But please note that for every ONE Devi Mahatmyam Parayanam, 108 Navakshari Japams have to be done once in the beginning and once in the end. For TWO Parayanams, it will be 216 in the beginning and 216 in the end. So please adjust your count of Navakshari Japam according to the number of Parayanams you do at a particular time. Below is the flow chart for this sequence.
Below is the procedure followed during group parayanams to help devotees leave after the first round.
In this method, Kavachaargala Keelakam, Raatri sooktam and Devī Sooktam, Rahasyatrayam will be recited only once in the beginning and end respectively.
In the First round, at the end of 13th chapter, Saptashati Nyāsam and Navākṣarī japam will be completed with Vimokam. Second round will start with Navākṣarī japam again followed by Saptasati nyāsam (bandhanam), 1st to 13th chapter, Saptashati vimokam, Navākṣarī vimokam. Same as above will be repeated for the third round.
After Navākṣarī vimokam, Devī Sooktam, Rahasyatrayam till end will be chanted and this completes the entire Pārāyaṇam. Please check the flow chart below for more clarity.