Do not plan on it. Because joining family members are considered in the initial application, their US$15,000 processing fee and US$1,000,000 gift belong to the same case rather than to their travel date. The fee is nonrefundable, and the gift is requested after successful vetting when instructed. Confirm the current sequence before budgeting around a delay.
Budget for the whole named household
Total the per-person amounts for everyone who will be in the application, then add the costs a staggered move creates on top: two households running at once, repeated travel, storage, and duplicated professional or medical fees. The gift is neither an EB-5 investment nor refundable equity, so it should not sit in the plan as a recoverable asset. Decide what the household would do if only part of the money proves available.
Add one further line to that budget: what a staggered move costs if the second stage does not happen on schedule. Two households running in parallel is the visible cost; the less visible ones are a lease renewed because a decision could not be made, a school year committed to in the wrong country, a spouse's employment gap extended, and travel booked repeatedly against dates that keep moving. Price a version in which the second stage takes twice as long.
Hypothetical example: a family budgets for a six-month separation and models a twelve-month version on the same page, and the second version changes which parent moves first rather than whether the plan proceeds at all.