How to merge columns and custom position widths

I cloned spotlight block and made a new one 'showcase' which can have 4 positions: showcase-1, showcase-2, showcase-3, showcase-4.

I need to use only showcase-1 and showcase-2. By default they both are width 6 (12/2) and I need them to be showcase-1 = 8 and showcase-2 = 4.

How can I set a different width than the default division of 12 / column # ?