I'm waiting to see if the IRS comes out with any regulations on the subject but 50/50 is my understanding from the law:

https://www.law.cornell.edu/uscode/text/26/6428

6428(e)(2) Joint returns
In the case of a refund or credit made or allowed under subsection (f) with respect to a joint return, half of such refund or credit shall be treated as having been made or allowed to each individual filing such return.

This appears to me to be another "big enough to drive a truck through" loophole that comes from poorly written legislation.  There's also the "every other year" dependents from divorced couples where one parent may have gotten the EIP money based on the 2019 return but the other parent gets the RRC in 2020.