Book a hotel for 1 person but a second person is coming anyway?

Hello, my girlfriend and I are celebrating our anniversary soon. Of course, we want to get physical, but neither of us has the house to ourselves. I've been thinking about renting a hotel. The best option in terms of location (location is the most important criterion for me because I have a date plan) is €38 for two people, and €19 for one person. The thing is, I'm reserving the night, but we're only staying from 4:00 PM to 5:30 PM or so. So I can basically book a room for one person (me), and they'll be my guest from 4:00 PM to 5:30 PM. Is that possible? Because I'd rather pay €19, of course.