Yes the front and rear video segments always start and stop at the same time, they are also named the same ( time / date ) and then tagged with A or B so you can tell them apart and you cant have 2 files with the same name in the same directory.
There are no way to play 2 files at the same time with a regular media player on a computer other than play them manual in a smaller window, you can give dashcam viewer a try it have a functioning demo so you can try it out / play around with it.
https://dashcamviewer.com/
You do not have to delete files at all, when the memory card are full the dashcam will automatic delete the oldest recordings on the memory card to make room for new ones. This is why having a memory card of decent size are important.
It is a good idea to once in a while format the memory card but you only need to do so every 2 - 3 months ( this restore the file structure on the memory card as it can get a little fragmented over time from all the writing and deleting )
Formatting in the camera mean everything on the card get deleted, so make sure to save anything you might want to keep before, you can also format / delete all the files om the memory card on the computer, but then its still a good idea to format in the camera also.
I run a 128 Gb memory card in my SG9663DC and i have several weeks on it, but i dont drive that much maybe 3-4 hours every week.
Keep in mind though the camera record all the time, its not like you have to save it all too, i personally say 99.9 % of what my dashcams generate are "garbage" as its just me driving and nothing happen, so really its only in that rare occasion of a event you want the camera to capture it and save it as proof for your insurance company or maybe police.
So when most of the stuff the dashcam do is "garbage" then its only right to let that get recycled so the memory card are ready for storing the important events, and lucky this is one of the defining features of a dashcam, and it is all automated so we users dont have to bother our poor memory with such things.
On the SG9663DC its a good idea to press the event button on the camera when you want to save something, this put the files in a folder named RO ( Read Only ) on the memory card, and files in that folder do not get deleted even if they are very old, you can only get rid of videos in the RO folder by deleting them on the computer or format the whole memory card in the dashcam.