No, it's not illegal at all. A company has a right to set terms or conditions on employment and to decide how to spend its money; these two factors together give it ample power to say that employees must double up on business trips and also that the company will only pay for one room for two employees. If the company is discriminating or harassing you on the basis of your sex/gender, age, race, religion or disability in some fashion in how it treats you on a trip (e.g. male employees get their own rooms; woman have to double up), that may provide a discrimination claim, but otherwise the company can do this. If it makes you sufficiently uncomfortable, you can always ask if you could have your own room if you pay the cost thereof, if that is worth it to you.