So I did some searching and I can find absolutely no mention of this from official sources (ie from Santander or JP Morgan). Coindesk sites zero sources in it's article (except for citing itself, because that is credible right?), and the only other articles I've found on this cite Coindesk.
I did find earlier articles saying that JP Morgan would be developing on ethereum from some more reputable sources dating from October:
http://www.businessinsider.com/jpmorgan-is-developing-a-new-blockchain-project-2016-10But guess who they cite? Yup, Coindesk. They also cite the WSJ, but that is behind a paywall so I can't confirm any of their info
And there is nothing to back up this recent article, nor anything to confirm that Santander is now involved, nor even any evidence that JP Morgan is following through with their plans from October, which was poorly sourced as is.
This one article is not enough for anyone to take this seriously, and looking through other coindesk articles I can see that they rarely ever cite their sources, this whole thing reeks.
Two sources I've found that talk about earlier bank lead blockchain projects
http://www.huffingtonpost.com/david-seaman/ubs-santander-announce-bl_b_11683838.html - "Mr. Whelan explained that even though the project uses Ethereum technology, it is not exposed to the public Ethereum blockchain. London-based blockchain lab Clearmatics, involved with the settlement coin concept, started with a flavor of Ethereum but this project is likely to end up being rather proprietary in nature." which means that they will likely not use the ethereum blockchain in the end.
http://www.reuters.com/article/us-banks-blockchain-ubs-idUSKCN10Z147 The most reliable source I've found writing on this, doesn't even so much as mention ethereum.
Until we see official word from either Santander or JP Morgan on this, this should be treated as false information, and even then it likely wouldn't mean they are involved for any reason beyond research purposes