To file as a "Qualified Joint Venture" (not sole proprietors) on two Schedule Cs they can just file the Schedule Cs.  There is no need to notify the IRS.

https://www.irs.gov/businesses/small-businesses-self-employed/election-for-married-couples-unincorpo...

As a side note, a Qualified Joint Venture can be done in any State.  The restriction is that if they are the only two members of an LLC, then that can only be done in a Community Property State.