This is actually a quote from a biblical passage, 2 Samuel 1:23. A longer version of the quote goes like this: "Saul and Jonathan were lovely and pleasant in their lives, and in their death they were not divided: they were swifter than eagles, they were stronger than lions" (King James Version).
Saul and Jonathan were a father and son. Saul was a king and both Saul and Jonathan died during a battle – Saul committed suicide after losing the battle and Jonathan was killed during the battle. The quote used for Eliot’s epigraph was actually spoken by David, who later became King of Israel.
Eliot’s allusion to Saul and Jonathan gives some interesting insight into the brother and sister duo, Tom and Maggie, to whom the quote refers. Jonathan and David were best friends, but Saul didn’t like David and disapproved of their friendship. Sounds a bit like how Tom disapproved of Maggie’s friendship with Philip. However, Saul was known as being very passionate, like Maggie. As you can see, there's parallel between Saul and Jonathan, and Tom and Maggie.
What’s also interesting about this epigraph is the emphasis placed on death. First off, Saul and Jonathan didn’t actually die together – they had two separate deaths that happened around the same time. But David’s quote refers to "their death" as a single thing. Secondly, this quote emphasizes how Saul and Jonathan were not divided in death, as opposed to how they were in life. This definitely reflects Tom’s and Maggie’s relationship, which had a lot of problems while they were alive. Ultimately, the siblings were only really "together" when they died.