I’ve watched videos where 3 holes were used in an asymmetric pattern so that the work could only be placed the correct way when flipped. I never did wrap my head around that but I have a way to do double sided that works for me. It does rely on the fact that I have the limit sensors and can home my machine accurately.
What I have done is found the center of my spoil board and saved that position to a workspace that I don’t work in, G59 in my case. I have also written the center coordinates down in case that workspace gets messed up. So I can always home my machine, go to G59, go to XY zero, then back to G54 and set XY to zero. Now my machine is in the center and zeroed.
From there I drilled 4 holes in a rectangular pattern, one hole for each corner of the work piece, with the center of the rectangle being the center of my spoil board. I then clamp the work piece so that it is centered and covering the holes in the spoil board. Then I drill the same 4 holes in the work piece all the way through and insert the dowels to make sure that everything has lined up correctly.
At this point I can take the work piece on and off the spoil board as many times as I need to. The only thing you have to watch out for is to always flip the work piece in the same direction as you did in the CAD program, either sideways for front to back.
With this method I have done projects where I have milled the front, then milled the back, then returned to the front to do some laser work after sanding etc. I can also turn the machine off or lose my zeroes and still recover. The only downside is it does rely on having limit sensors so that I can reliably get back to the center position. And as a bonus if you do another project that is the same size you can reuse the holes in the spoil board. I also saved the program to drill the holes and wrote down the coordinates for the holes just in case.
Maybe this way will work for you or someone else if you have the limit sensors.
EDIT: I should also add that I use the center of the work for the origin when carving, not the lower left corner. And also make sure you have everything else done before you cut the profile and lose the holes as there is no coming back after that.