Well, I'm quite sceptical whether the ECA number or other details for spouse could be updated later! You're supposed to submit a complete profile beforehand to claim the ITA, because when we get the ITA, the profile gets locked and all of our information is stored at back end, so making alterations afterwards would make your profile prone to investigation.
Thus, I will simply recommend you to wait for your spouse's ECA first, only after getting it submit the profile. Remeber "it is better to wait than to be stucked! "
Well, you can add your own ECA now and update your profile later. Just that you would have a different score and depending on the draw you either get the ITA or not. From the initial question, the person does not have the ITA yet and we are still talking about the profile creation.