The only example of a second encounter joining the first, that I've run, ended up with the party retreating. It was from the KOTS, but it wasn't the Iron Tooth fight. It involved the Warlock open up a door to an "empty room", where a solo eventually came out of. The party had wasted a number of encounter powers clearing out minions, before the Elite was able to make it to the fight ... and the Solo being added in didn't help. Since it was a pair of oozes, it wasn't hard to run away when they saw it was a problem. They had to go back and fight the combined solo and elite fight when they got back, but they knew what they were up against.
The way that relatively easy encounters can be combined into a single hard encounter is a good way to design areas to reward players for their tactics. It gives you the ability to have a hard encounter where the individual monsters aren't that hard (since you wouldn't necessarily have them above the PCs level, so they aren't hitting too often, or not getting hit often enough), plus the rolling waves of monsters makes it a bit easier for the PCs to handle (assuming they don't make tactical mistakes like using dailies or encounters at the right time). For PCs with encounter long powers (flaming sphere, barbarian rage, etc) it can come in very handy.
In a campaign I've been playing in, we had a TPK encounter that was a combination of issues. It was near the beginning of the SOW. The combination of no one in the party bothering to crack a sunrod, the inability of the defenders to get over/out of pits, my warlock failing his stealth check to "scout ahead" around the corner, and the fact that it was a combination of encounters (the people attacking in the dark hallway fallback to a second encounter, which gets a couple people from a third encounter, not sure entirely.) I just know that we went through the encounter once, it ended in a TPK. It was "just a dream", the second time a few things were changed, but it still ended up with a few PCs surviving via stablization by the monsters (in an attempt to take us alive).