From a strict ledger of transactions metaphor of the blockchain they could be joined without double value but it'd require that all blocks from both chains were

From a strict ledger of transactions metaphor of the blockchain they could be joined without double value but it'd require that all blocks from both chains were strictly disjoint sets of transactions and no account overspent when combining both chains. On a more real technical level though the protocol doesn't have a way to have two parent blocks even if the two chains satisfied the conditions to avoid double spending.