Tiling relies on 2 things to be successful:
1. Software layout
2. Machine/Material setup
So many ways to approach this. The following is my method of tiling and has generated very good results for me with minimal indexing holes.
Software
Assuming you are working with a Vectric product, the tiling feature works just fine. However, I prefer to control where I break a file so I don't use the tiling feature.
Instead....
I created a sign 18" x 72" and used center of material for my XY Datum
Converted text to curves and then created 2 bounding boxes (rectangles) beginning at the "O" location you suggested.
Copied all vectors to 2 layers....Tile 1 and Tile 2.
Display only Tile 1 layer and use the interactive snipping tool to cut the "O" that is outside of the bounding box and delete the remainder of the vectors outside as well.
Display only Tile 2 layer and use the interactive snipping tool to cut the "O" that is outside of the bounding box and delete the remainder of the vectors outside as well.
At this point you should turn on both layers and see the 18" x 72" sign still intact.
Create a center line at the midpoint of both bounding boxes.
Create 3 register holes and place 1 on each center line and 1 at the center of the material (this one will be milled in the spoilboard only).
Turn off all layers but Tile 1. Copy this to a new layer named Tile 1_Cut. Select all the vectors on this new layer and center it in the software.
Now your ready to join that "O" that we cut earlier. I used the join tool with smooth curve. Do not join the "O" earlier in this process or each tile will be offset when you center the vectors.
Repeat with Tile 2 layer.
That completes the Software side of things.
Machine/Material Setup
Attach a board along the Y axis the full length of the cutting area on your machine.
Use an endmill of your choice and plane/joint 1 edge so that you will have a perfectly aligned edge to the Y axis. I use a 1/2" endmill myself and simply manually jog the spindle up the Y axis with the HHC.
Draw a center line on this board. In your case that would be 24" from either end.
Draw a center line across the width of the actual material being used. In this case I am using a piece of wood 18" x 72" so that line will be 36" from either end.
Draw a small center line 9" in from either edge across the line you previously drew. This will become your XY-0.
Place your material on the machine butted up tight to the board that was made making sure to align the center of material with the center line of the jig.
Set your XY-0 to the center of your material being used. Cut your 2 indexing holes in that material and then remove the board and cut the final indexing hole in the spoilboard.
Now you can cut Tile 1 by placing a dowel in the hole made in your material and into the hole made in the spoilboard.
Repeat with Tile 2.
As stated earlier, this is my method and it works for me. The vertical board on my machine stays put and I use it over and over for various jobs. I have successfully done this with up to 3 tiles to date without any issues.
I've attached my file for dissecting.