<p>The article considers the issues of secure functioning of a two-level blockchain with a complex mixed consensus protocol, namely, Proof-of-Work in the main blockchain (mainchain) and Proof-of-Stake in the secondary blockchain (sidechain). The principle of building such a blockchain is based on the Proof-of-Proof protocol, where a stable blockchain (mainchain) is used to ensure the stability of the sidechain by referring the mainchain blocks to the sidechain blocks using special transactions. Such a structure allows for faster block generation in the sidechain and, accordingly, faster processing of transactions without reducing stability and without increasing the block size. In turn, such a two-level blockchain is of the greatest interest for the creation of a cascade system of state registers, which will guarantee the protectection against the substitution and forgery of documents. The main results of the article are explicit analytical expressions for estimates of the probability of double-spending attack on such a two-level blockchain under the condition of an adversary in the sidechain and in the mainchain. The obtained expressions allow finding the number of confirmation blocks in the sidechain, which guarantees security against the attack with a probability no less than a preset value.</p>