The first step should be going on a walk with both dogs and at least 2 people, each holding one dog. Start walking with both humans in the middle and the dogs on either side. Just walk like that for at least 5 minutes and keep the dogs seperated. Once they are calm and in the mood for walking, you can allow the dogs to walk side by side or play with each other.
What this does is showing both dogs that the other one can and will behave in a social manner. It also puts them together in a pack, since you all (dogs and humans) form a social group that works (or rather walks) together. This is the first step of reconciliation.
Next is to seperate their food bowls. Food is the number one reason for aggression in multi-pet homes, especially if both pets don't know each other very well. Either feed them at seperate times or in seperate rooms.
And last but not least, know the hierarchy. Both of them have a clear understanding of who is higher ranking than the other and they communicate this to each other via body language. If you happen to give both of them a treat, but you offer it to the lower ranking dog first, the higher ranking one might become aggressive. In the natural social structure of dogs the highest ranking one always eats first, so by eating the treat the lower ranking dog challanges the dominance of the other one, who then starts a fight.
If you're not sure which of your dogs is higher ranking, offer both treats to both dogs at the same time (one in each hand).
Bonus: Do some obedience training with both dogs at the same time. That can be as simple as "sit", "lay down" and "come here", or more complex commands, depending on what they know. This puts them together as a pack and can be a pleasant situation (if they get treats for following the commands). Don't forget to give the treat to the higher ranking dog first.