Are you modeling a single part, or mating two parts in an assembly?
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Either way, start out modeling the 1X10 and 2X4 rectangle as a sketch centered around the origin.
Draw a rectangle.
Draw a reference line from the lower left corner to the upper right corner.
Contrain the midpoint of the reference line to the origin.
Dimension the rectangle as appropriate.
Extrude the rectangle the desired length by the midplane.
If a single part, start on the face of the box you just extruded and sketch/extrude the second box in the same manner.
If an assembly, create a second part in the same manner as the first box, i.e. centered about the origin and extrude via midplane.
When you mate the two boxes together, you align the part's planes and they will be centered.