The shield other spell works off half the damage so it continues to be useful at higher levels. Life link not so much, true. Tho' it's hardly alone in 'things that are useless at L20', much of the game's broken by then.
Even if you cannot double up the links it still continues to heal your companions. So while you may not be able to take full advantage of having 20 links it still continues to work. Taking 100 HP of damage per round may be a bit much even at 20th level. Unless you are using something like heal every round taking that much damage has a good chance of killing you or forcing you to drop the links.
Well if you got Life-Link from 2 different classes I assume you could stack them. Certainly if you had 2 Oracles in the party and they both linked with the same Barbarian that Barb would heal 10hp and both Oracles would take 5 damage.
It's worth noting that this could go badly for you sometime though, if the Barb is on full health and takes 5hp worth of damage it would trigger both Life-Links. The Barb would heal back to full (5hp) and both Oracles would take 5hp of damage.
(Also I don't know if any other class can get Life-Link, but if they can I assume it'd work.)
The spirit guide oracle can get life link as an oracle and as a shaman. They should stack, especially since they aren't identical abilities, just very similar.
Life link is mostly about convenient damage distribution. It makes your channels slightly more efficient. Shield other is similar, but you don't want shield other on more than 2 allies, because you could easily die to an area attack. (Honestly, even 2 allies is a bit much for Shield Other, but your odds of surviving are still ok.) Life link is for topping up allies who don't really need much in-combat healing, while you focus on keeping yourself and your most damage-prone allies in the fight.
PS: The spirit guide double life oracle can also get double sources of channel. It's really the best pure damage healer in the game. The Pei Zin is almost as good at damage healing, but much better at status removal, which might still make it a better overall healer.