While joint talks are OK, we generally discourage it. It's hard enough for one person to find his or her rhythm on stage and to really engage the audience. Two people rarely get truly in sync to present something compelling. In our experience, when two speakers are on stage at the same time, they may draw attention away from each other and struggle to clearly explain their topic to the audience.
If you do host any talks featuring two speakers, make sure to watch the talks with more than one speaker that we've featured on TED.com for inspiration. Caitria and Morgan O’Neill is a good example. Handspring Puppet Co. also did it well -- but their demonstration is unique and hard to emulate.