Remove partition and restore the space without erasing

Time ago I split the hard drive (500gb) in two partitions (250 each), now I don't need them anymore, so I have first erased successfully one partition and now I have still the second one (Mac HD 2)

See the image attached

What I need now is to reapply the 250 gb I have gained from deleting to the Mac HD2, how is it possible? I am in the situation that I don't want erase it but I need the 250 gb left there.