I am looking for authority to support the investment of one Rev. Rul. 81-100 collective investment trust in another Rev. Rul. 81-100 collective investment trust. Such an investment is not covered by Rev. Rul. 81-100 or 408(b)(8). Any help? Thanks.

See DOL Advisory Opinion 82-41A, interpreting 408(b)(8). That addressed a similar situation. There may be additional authority available, but that's the letter I could find in my file first.

Although it might not be the kind of obvious public authority that you seek, here's another way to look. It's common for a sponsor of a group trust to include in its trust declaration that the last of the kinds of permitted users is another group trust that has an IRS determination and restricts its use to the same kinds of retirement plans and other eligible users that could directly use the group trust. In my experience, such a provision doesn't cause any difficulty with getting a "clean" routine IRS determination that the group trust is a tax-exempt 81-100 trust. So one starting point might be to look at the text of each group trust's governing documents: does the "investor" group trust authorize investment in another group trust? does the "investee" group trust permit another group trust as a user?