By operation of law, the beneficiaries become responsible for any portion the year-of death RMD of the decedent that the decedent had not already satisfied. Based on CFR 1.401(a)(9)-5 Q&A-4 saying "that amount must be distributed to a beneficiary" [emphasis added], the year-of death RMD can be taken by the beneficiaries in any combination. For example, if one beneficiary cashed out an amount sufficient to complete the decedent's year-of-death RMD, the other beneficiaries need not take any distribution for the year of death. The distribution taken by each beneficiary is taxable on that beneficiary's tax return.
If the beneficiaries cannot agree on a combination that completes the year-of-death RMD and some portion of the RMD goes uncompleted, the IRS is likely to treat the responsibility for the total amount required to be taken by the beneficiaries as being in proportion to each beneficiary's share, but there is no specific guidance on this. You can probably argue that any amount taken by a particular beneficiary above their proportionate share proportionally satisfies the proportionate shares of other beneficiaries, but again there is no specific guidance on this. Say there are four equal beneficiaries and $12,000 of the year-of-death RMD remained unsatisfied by the decedent. I think that if one beneficiary received a distribution of $6,000, the other three beneficiaries would be responsible for taking $2,000 each unless one of the remaining 3 wanted to take more. However, if three of the beneficiaries took only $3,000 each and the other beneficiary took nothing, the beneficiary that failed to take a $3,000 distribution would be the one responsible for the RMD shortfall.
There is no explicit accounting of RMDs on tax returns, just the reporting of the distribution the same as if it was not an RMD. I would retain any records that substantiate that you took whatever portion you were responsible for taking, including information documenting the amounts taken by other beneficiaries if the RMD is not satisfied proportionately.
(Having 20 beneficiaries certainly complicates coordination of satisfying the year-of-death RMD, but I would expect that any beneficiary that receives a distribution of a proportionate share of the RMD would be safe.)
