I have never seen any guide to explain what to do if you have only one Scout around because the other one is using an utility class.

Does this makes changing to other utilities (Say Pyro, for example) more viable? Do you prefer to exchange a Soldier for that Sniper and keep both Scouts around? How important are the two scouts?

I need to instruct my team about this but, frankly, I don't know how.

If you're running a single scout, your main concern should be flanks. When two scouts watch a flank, they can hold off the enemy scout pair fine, but a 2v1 is oftentimes too demanding. In many situations it's good to have the demo or roamer provide support or be ready to back-up the lone scout. Your single scout should be ready to retreat if he's by himself (if you can, see if you can get a trap set up and have the lone scout bait). Your medic should be willing to put a buff on him fairly frequently, as he'll be taking the brunt of any flank harass. The lone scout also needs to be aware of his heightened responsibility and make calls whenever someone slips past him or he loses sight of the flank while retreating.

The double-scout importance is very dependent on the map and situation. Final-point standstills in particular are usually better suited to sniper/spy than scout, if they can get there in time and have the practice.

Switching out a soldier always runs the risk of getting your team out-spammed, or if your pocket goes down, it could leave your medic more prone to picks. There are always exceptions, like running pyro or heavy on gpit C or a roaming pyro on blands mid, but you should avoid trying to apply those in too many situations. Pyro and heavy work on gpit C because you never need to worry about pushing or horizontal combat, and blands pyro works mostly because the closet position lets him contribute to the midfight spam, something he normally can't do.

In regards to swapping out a soldier for a sniper, it's entirely dependent on the map, I'd never take a soldier away on a map like badlands for a sniper, the ability to add covering fire (read: spam) is something absolutely vital to the mid battle on that map. A scout maybe, but a soldier is much easier to take away on a map like Yukon where 2 scouts are great and soldiers a little more limited.

In Single-Scout scenarios, I change up and go from engage flank directly and try to lure the scouts to where my sollys/demo can get some heat on them, or simply keeping the combo aware of their location. You most likely not going to win a 2v1 against 2 scouts, so pushing into it is somewhat of a waste, so you need to try to keep them at a distance and not take to much damage while doing what damage you can until one of yoru heavies can get a shot at them.

When swapping a Scout for a utility, a lot of the times I find it useful to have the Roaming soldier run flank with him. Unfortunately both of them need to play more passively as the Scout may get himself stuck in a 2v1 situation, and the Soldier doesn't have the speed to retreat back to the combo like a Scout does.

So, if you have an extreme aggression team (read: mine) then you may have trouble on your flank. Chances are you'll have plenty of firepower to go aggressive from the front with the combo, so good communication with the Roamer/Scout when to go aggressive is key IMO.

In terms of swapping a Soldier vs Scout, I'm sure that topic can be discussed ad nauseum.

Feel free to check out this thread: http://commforums.com/swapping-out-scouts-versus-t2320.html?t=2320

It discusses swapping out Scouts vs Soldiers for a Sniper. But I feel a lot of the same points can be made for other utilities (Engineer, Pyro, Heavy, etc). For the most part it is largely dependant on the map and the current situation of the round.

everyone has a different opinion, but IMO it depends on a few things...

first, the skill level of your lone scout vs. the skill level of the other team. If you have a beast scout who can handle going all rambo against a team of equal, or lower skill then just let him do whatever he feels comfortable with. If the scout is not as good as theirs then he needs to play smart and wait for an opening to attack. Let a soldier, and maybe even the demo bomb in on the other team at middle, once they jump have the scout start to flank around. When the solly/demo land and deal out a good deal of damage the scout should be ready to pop out of nowhere and start getting cleanup kills.

second, it depends on the map. On badlands for example, if you get a good spawn you can sometimes get into the enemy teams yard as they are getting into the house (as long as they dont send anyone choke to spot you). If you are the lone scout you could get behind the medic before he ever expects it and take him down with a quick 2-shot. On granary there is no real way to flank around since at mid you have to go either left or right, right would be a death trap since its where the other team normally comes out. Left usually has someone watching it as well, so its tough to flank. Your best bet would be to hang back near the medic to watch for flanking scouts while the heavy classes spam the enemy corner. If you start to weaken the other team, or start to get a numbers advantage you can attempt a flank or move in on them from the crates, or down low.

So yeah... there is no one solid answer for that question, it depends on a lot of things.

Just play more carefully when you are alone and find another class to stick with, like maybe the demo or utility. Also 1 scout is sneakier than 2.