I've done this several times (Gosh, actually about 1 time for every level of play, thinking about it). It's fun; and it shakes things up bit. Also, it can simply seem realistic.
My advice (which I'm giving because my group has enjoyed every single time I've done it):
If possible, use a "wave". Don't simply combine two encounters, but, if it looks like two encounters may merge, plan on that from the first encounter, if possible. Have the first group of opponents consist of a vanguard, and a rearguard, who doesn't move to engage simultaneously. Then have some of them (maybe the survivors) move back to the next encounter, or have some of the next encounter move forward into this one. Give the PCs a bit of breathing room, if possible.
By the rules, they don't get XP or Healing Surges, as there was no Short Rest. However, let them know that this will count as two encounters once it is clear that it will. That way, the PCs know that they will get a milestone (and thus an Action Point and another Daily Magic Item usage). This lets them feel a bit more free to unleash cool stuff, and win the battle. If the end of the first encounter would have been a milestone, you could even give them the action point and daily magic item usage right then (though I still limit them to 1 per "total encounter"). Still, this can help recharge someone who was out of action points.
Be fair. Because the monsters are willing to step into another encounter, they should also be willing to "step out" of an encounter. If it makes sense for them to come into the fight, sometimes it should also make sense for them to retreat or flee from a fight. If the only decisions monsters make is to "hose" the party, it's going to feel unfair.
Adjust on the fly. If the party had a really easy encounter (few people are hurt, few dailies got used) then you can be pretty lenient in adding a new encounter without a rest. If the party really got bushwhacked in the first fight, only add a second fight if you absolutely have to.
Make it special. If you are going to add more monsters, don't just throw them into the mix. Have them charge in with battlecries, or mock their weak companions. Have them form a shieldwall, or rush in from a side passage. Since combining encounters should be relatively rare (though, again, it turns out I do it a lot!), it should feel like a special, cool event, and not just "oh, no! more friggin' hobgoblins"!
Msherman's idea was good, in that he came up with both a plausible reason to allow a bit of a rest, but also tied that into a chance for surprise. I really like that, and I'm going to steal it. Also, I've told people that they could start taking a short rest, and asked what the first thing they were going to do during the rest was (like, the fighter sat down next to his foe and examined his weapon). That way, they didn't get the chance to use Surges, but they can get a little bit of prep (like our paranoid ranger whose first action was to watch the door, so there went the surprise round for the second wave).