There are no household accounts in DC so it isn't possible to combine miles.

Remember though that you can make one-way redemptions so if you have enough miles in your account for a single one-way miles + cash, and your partner/friend has enough for three one-ways, that's enough for two round trips. You just need to book as one-ways from each account separately.

Alternatively, you can buy up to 20,000 miles each to top up your accounts.