If you ask me, since both Lafitte and Blackbeard have been talking about their "magnificent plan" in order to make Blackbeard the Pirate King. Therefore, I think that they will not have that encounter very soon, since I think Oda's put a lot of effort in this specific plan.
I also hypothesised that what happends will go something like this:
Blackbeard defeats Ace totally, leaving Ace on Banaro Island, believing Ace to be dead. The Blackbeard pirates then head for Water 7, searching for the Straw Hats.
A couple of days later, a merchant ship will arrive at Banaro, finding Ace, almost dead. They picks him up, first not knowing his identity, but when they realise that he's "Fire Fist" Ace, they see their chance of getting famous. Ace's still so wounded from his fight with Blackbeard that he's more or less unconscious. When he wakes up several days later, he cannot remember what happend, or where he was. After thinking back to it a lot, he realises what happend, and has only one thought in his mind: "LUFFY!". Ace thereafter steals one of the lifeboats on the merchant ship, realising that he has to find Luffy before Blackbeard does.
During the summing up of the Mermen arc, after it's end, Blackbeard arriveds in Water 7, realising that Luffy has left the island several weeks ago. Angry, he comes to the conclusion that following Luffy further would interfere with his great plan, as he does not have time for following the Straw Hats any further, and decleres to the crew that he setting sail towards the New World, since he's got an Eternal post to some island there.
Of course, as several times earlier, this is one of my random speculations, without any good arguments to back it up.
