I've done back to back encounters. I've done them with warning, and without warning. I've had them be optional with consequences for a short rest, letting the players know beforehand at times, and keeping them in the dark at other times.
I think it works best if the players know it's coming. Opening moves for some of my party is encounter power action point encounter power, during enemy turn immediate action encounter power. At the end of round 2 the player might be out of encounter powers. I won't discourage this, as it makes encounters go fast. But it also means I have a bored player spamming an at will for the entirety of a second encounter when they don't get a short rest.
I find ways to tax their action economy, or give them other actions they need to do when running back to back encounters. Skill challenge during an encounter puts a dampener on the encounter powers the PC's tend to use. I also will implement something in the environment that behaves like an encounter power. Perhaps a PC notices a loose branch hanging by a vine. Knocking it off might hurt a bunch of enemies and sweep them off their feat. There's an encounter power right there. A fighter might push a statue down on some enemies getting their attention, and distracting them from his allies. I also make sure these environmental options have at least the accuracy of, and do the equivalent of (or often more than) the damage of a PC's at-will powers, so they won't feel like using an at-will would be a better choice. (I utterly hate things like alchemist fire laying around for you to use that does +4 vs reflex, 1d6 damage, bleh).
I think occasional back to back encounters can add more interest to an encounter. Imagine a situation where the PC's defeat half an encounter, survivors flee, PC's chase them (quick skill challenge), upon success, they corner them, but the survivors manage to yell for their friends, making it a two wave second encounter, and upon failing the skill challenge, the survivors meet up with their friends, and the PC's have to take them all on at the same time, and have spent maybe more resources during the skill challenge like healing surges.
On the topic of "how to let the players know" it of course should use in game logic. I would shy away from telling players out of game that they will have 2 encounters back to back without a short rest. At the beginning of the encounter, I might say something like:
"As you charge the orc sentries, they raise an alarm, and you can hear heavy footsteps coming down to the gates. Sounds like they will be here within the a minute."
Or if the PC's have a time sensitive task ahead of them, I might say:
"Half the orcs retreat while the other half try to hold you off. You see a couple orcs getting ready to lift the draw bridge, If you quickly finish these orcs off, you may be able to get to the draw bridge in time, but that will put you face to face with the dozen or so orcs waiting there as well as the half dozen that just fled here."
Noticing, most the orcs are minions, the PC's may decide to go for the no rest option, hacking their way through the first wave, dashing across the draw bridge, and entering the gate way to fight the orcs there, at which point they see more orcs running to the battlements, and the PC's may realize they won't get a rest for a while.
I think the "must have short rest" constraint of 4e too heavily influences the scenarios, or "scenes" that can happen in the game. It's the DM's job to not let the scene be hindered by the mechanic. For instance, in the middle of the assault scenario above, one of the PC's may climb a tower, to knock over the orcish flag, giving everyone a huge morale boost, the equivalent of a short rest. During the fight, the orcs unleash a war troll upon the PC's hoping to weaken them, but they fear it's wild swings, and other than taking a few ranged pot shots, stay away from the fray. Once a PC knocks the troll down, boom they get the benefits (and hindrances) of another short rest. The angry orcs hesitate a moment before they charge after having witnessed the PC's prowess, which gives the PC's time to relocate and maybe take on the orcs at a bottleneck, waiting for another break to make their way to the battlements for a planned escape over the wall. This might complete their day 1 of the hit and run assault on the orc castle, leaving the half-orc rogue behind to blend with the orcs, and pave the way for the next day's sneak attack into the quarter's of the orcish commanders.
My adventure designs have so far been heavily influenced with the short rest requirements, and I really need to loosen that straight jacket and allow more flexibility. Back to back encounters (with in game warning, and environmental encounter powers) is one way, and giving the benefits of a short rest at scenario points is another.
