How can I define multiple instances in molecule which differ only in name?

I've got a molecule.yml which looks a bit like this:

dependency:
  name: galaxy
driver:
  name: docker
platforms:
  - name: testohpc-compute-0
    image: docker.io/pycontribs/centos:7
    pre_build_image: true
    groups:
      - testohpc_compute
    command: /sbin/init
    tmpfs:
      - /run
      - /tmp
    volumes:
      - /sys/fs/cgroup:/sys/fs/cgroup:ro
    networks:
      - name: net1

How can I define another instance, say testohpc-compute-2 which is exactly the same except for name? Do I really need to copy all the definition from -1 again?

Furthermore, if there's a way of reusing an instance definition, can I share it between scenarios?

You can take advantage of yaml anchor and merge key features. You can find a basic explanation on Learn yaml in Y minute.

In your specific case, here is a possible solution.

platforms:
  - &default_platform
    name: testohpc-compute-0
    image: docker.io/pycontribs/centos:7
    pre_build_image: true
    groups:
      - testohpc_compute
    command: /sbin/init
    tmpfs:
      - /run
      - /tmp
    volumes:
      - /sys/fs/cgroup:/sys/fs/cgroup:ro
    networks:
      - name: net1
  - <<: *default_platform
    name: testohpc-compute-2

Note: anchors and merge keys can only be used in the same yaml file. So this will not work between different scenario.