A Precise Table Saw from an Electric Hand Saw

Step 12 Make a larger shim

The second slot should be at least 12 inches off to the side of the first slot.  You can make your own shim to offset the slot, or you can buy something that is reasonably certain to have parallel edges, like a piece of shelving.  After the first cut for the slot has been made, use the smaller shim to cut the outer edge of the second slot, just as you did on the first slot.  This allows you to leave the square undisturbed and gives a greater probability both slots will be exactly parallel to one another.  (Note that this drawing is not to scale.  The width of the shim (shown in shades of brown) in this graphic would be much greater than shown so that the second set of cuts would be much farther away from the first set.)