Just a few ideas:
a garden bench - you would need legs, maybe the second piece could be cut with a cement cutter (rent one?) to make the legs
a retaining wall if you have a slope, or stairs/steps
could they be cut for stepping stones? pedestal for bird bath or fountain?

Do you know if these were reinforced? Sometimes concrete counter tops are not, they could have expanded metal lathe, or even pencil rebar,, etc.That known you will know the strength of them as well as how well they may handle the weather.Also if they are going outside remeber to seal them properly as countertops are not typically sealed to handle outside.
I have used similar pieces for a base and background for statuary.Leaning the larger one upright the smaller one as a level surface for a piece of statuary.growing vines are it to soften the edges.I have used these for a small tables having them placed on large stones or stacked bricks, as well as haveing cutome legs made by a metal smith.The larger one would also be great placed as a small landing in the garden or even in front of a barby.
they also make a nice flat surface to work mosaics on.you would have a nice background if you did a design in the center and corners,,I am sure you will figure out something they will be ideal for.
M