I agree. Don't do this. It's not fair. It's dishonest. It's probably illegal too. Besides, it's not likely that you'll be able to sleep two in that single bed - they're even smaller than in the US (for example). They might well allow you both to share a single room but since there will be two people you'll surely be charged for a double so what's the point? Even with a double room you'll MOST likely have a room with two single beds either apart or pushed together.

There are a few VERY cheap "hostales" where a double could be around 20-25 Euros. It's actually cheaper than the two of you staying at a youth-hostel which would likely charge 15-20 Euros per person AND you wouldn't be able to share a bed or the same room!

Saludos, MadridMan