Convincing siblings to get along can be a battle. For them to get along and work in chores together might require a lot of patience and practice. Rather than emphasizing the importance of working together, show the kids that working with their brother or sister has advantages. For example, they can complete the chores more quickly and easily that way, so they can go back to playing.
Set up a chores schedule and plan for them to share some of those chores. If the kids won't choose to work together, set it up so it happens anyway. With time, they might discover that working together has many advantages. Let the kids work out their own arrangement. For example, designate siblings to work together on setting up the table, and let them figure out who will set the plates and who will put the glasses out. This allows children to learn to share responsibility.
Set up a payment arrangement for chores. Rather than giving your children a set allowance, pay them for completing specific chores. Create a list of what each chore pays, and pay them more if they complete a chore together. For example, watering the garden pays $1 per child, but if the kids work together on it, they'll get $1.50 each. This teaches children to earn the money through work rather than expect a weekly allowance no matter how many tasks they complete.
Ask an older child to teach the younger one how to do something. For example, a 7-year-old should be able to make his own bed easily, but a 4-year-old might have trouble with it. Rather than you teaching him how to do it, let the responsibility fall on the older child, so they can work together on it.