I'm gonna go ahead and disagree with what the others have said. Depending on use & distance, people of differing heights CAN share bikes. It may only be anecdotal evidence, but MY WIFE & I DO IT. She's a good 3" shorter than me, has shorter arms & legs.
If you are looking for a bike that is comfortable for the both of you, I'd recommend something that is more upright (hell, I'd recommend this anyway) than racey. Reach won't matter as much & as long as you can both stand over it, you're fine.
We share a Yuba Mundo for kid & grocery hauling, though, I wouldn't recommend it as the only bike for you. We also share an 80's Schwinn High Sierra. It's too small for me in most eyes, but it's comfortable w/ the right stem/bar combo even if it has 12" of seat post sticking out. Old mountain bikes are great for city commuting & if you get the right size, they can work for two people.
So, it can be done. My question would be, though: WILL YOU EVER WANT TO RIDE TOGETHER? If you wanna get groceries together, or go to the park or even ride in seperate directions on the same day, I'd say get two.