For me this would not be an issue from the "will he have sex with someone else perspective," though if that is what is in the back of your mind -- you and he have other things to work out that are bigger than this trip. Can you step back and see that? Does he have something in his past that makes you leery? Does he like to drink and you're afraid he'll make a fool of himself at best or get into trouble at worst? Does this couple seem to bring out a side of him that you don't like much?
As an adult, I would not take a trip that I could not afford, period. This one sounds a bit more like "let's do this for fun and it can count as 'work' too" than it sounds like a very serious conference -- which is the case?
I can see sharing a room with my (female) best friends on a fun girls' weekend trip, but not what you describe. If he's doing this to save them money -- not because HE is strapped for cash -- it's not his responsibility to save them money. If he's doing it because he too can't afford this trip without sharing, then he should reconsider doing the trip at all. Is this trip actually essential to his degree, to getting ahead in his career field, to meeting people who can help him land jobs someday soon? Or is it more "It would be a fun reason to go to city X for a few days"? If the latter, forget it, but if the case is that this is legitimately a business trip he needs to take for professional/educational reasons, then he should treat it like a business trip and get his own room. Try a cheaper hotel, find deals online (there are lots), etc.
Their attitude says a lot. I'll be the first to say that on many business trips in the past I watched a movie or two in the hotel room, or went out and enjoyed the local sights, but it sounds more like they're young and looking at this as an excuse to hang out. If they are indeed pretty serious students, well, they may not have meant it as "teenagery" as it sounded. But you and your husband have to make the call here; he knows whether they're serious students or are going mostly to have a trip and some fun.