There is often a big difference between dogs on either side of a fence. The fence seems to build confidence in a softer dog so that he will arc up at another dog on the other side of the fence. See it all the time with the dogs exercising in the yards here.
Same applies to dogs on lead so my suggestion is you introduce the dogs on lead but do not hold the leads tight and only let them smell each other for 3 seconds then turn and walk away a few paces, turn around and come back again, meet for 3 secs and part. Do this a few times.
This will give you an opportunity to judge the temperement of both dogs and allows the dogs to judge each other. If there is any growling or hackles come up, they probably won't get on. If tails are wagging and ears are up, both dogs look happy, try off lead but be ready to intervene if needed.
Border River Pet Resort