An RPC node runs the witness plugin and the blockchain file grows on all nodes even seed nodes. They cannot share the same file.
While docker images can easily share data, that does not mean the underlying applications can.
Two diverse internet providers would provide completely different IPs.
I thought you were talking about sharing witness and full node on same hardware, now you are talking about two ISP?
HEDT motherboard often have two Ethernet Ports. Internet via two separate ISPs can plug into each port and each VM or docker container can use a different ISP as main and the alternate as backup. Only in event of outage of one would witness & API nodes be using same ISP & IP address range.
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