Depending on your system you can copy the blockchain manually from the first node to the second. You should stop all bitcoin or lightning services before. Seach for copy blockchain and your setup name eg. raspiblitz nodl umbrel etc. on google.
I recommed just letting the new one sync for lazy or inexperienced users. I think @DarthCoin here knows how to umbrel for example ;)
I know and I did that manual copy many times in the past. But I do not want that. I just want a new node to be like a "child" to the old ones and instead of going out in internet to use local nodes. That's all. So no interruption of existing nodes, just a smooth local sync. Once is done, to switch back to "normal" sync. I know it is a bitcoin-cli command but can't find right now. I remember I did something like that long time ago with Bitcoin Core/QT app, pointing to another node in LAN and grab the blocks from that local IP.
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Ok then you could try "bitcoin-cli addnode" commads for the new node and add the old one local.