You have to add the second registration for the same person after you've completed the first race registration. You can do it all in one transaction though (i.e. you don't have to leave the registration page and go back into the queue). You'll see an image like below:
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You'll only be able to click on one of the choices. So you wouldn't be able to click 10k and HM simultaneously. That's because they have two slightly different sets of secondary questions. So do your 10k registration, then at the bottom of the page you'll see:
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"I want to add another registration" and once you've finished your 10k registration, you can then add your HM registration. After that you'll go through a third time to add your DH's 10k. Once all three have been filled out, then you click "Next Step" and are taken to the pay screen.