Upon the death of the decedent the beneficiaries became responsible for completing the decedent's 2019 RMD, so the decedent is not the one subject to the penalty. The IRS has not provided specific guidance on which beneficiary will be subject to the excess accumulation penalty if the full RMD for 2019 is not completed, but it's commonly believed that as long as a particular beneficiary receives a distribution in proportion to their respective share (50% of the RMD in this case) that that beneficiary will not be subject to the excess accumulation penalty. This means that it would be the uncooperative beneficiary who needs to file Form 5329 Part IX and pay the excess accumulation penalty on their 50% share of the RMD.
Section 1.401(a)(9)-5 Q&A-4 just says that the RMD must be completed by "a" beneficiary, so if the cooperative beneficiary wants to be absolutely certain that they cannot become responsible for any the penalty that beneficiary can take a distribution of the remainder of the RMD.
