If you use a BIP-39 passphrase, the letter has to say one exists and where to find it. Every passphrase produces a valid wallet, so the 12 words with a missing or wrong passphrase open an empty wallet with no error, and heirs may conclude the coins are gone.
On splitting: the standard way is SLIP-39 (Shamir's secret sharing). You pick a threshold such as 2-of-3, and holding fewer shares than the threshold reveals nothing about the secret. It uses a different word format from BIP-39 and the two mostly aren't compatible, so the letter should name which wallet restores it.
Two additions that often trip heirs up:
Sources: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki, https://github.com/satoshilabs/slips/blob/master/slip-0039.md