Alternate Universes in TaMD/WR
Existance is a big place, with a lot of possibilities. Amber is solid proof of this, casting Shadows of itself to form everything imaginable. It therefore should not come as too much of a surprise that there are, perhaps, an infinite number of Ambers, all distorted reflections of... what? No-one is really sure. Perhaps the higher-order Pattern in the Jewel of Judgment.
Regardless, there are an awful lot of alternate realities out there. And you thought Shadow was confusing...
How does all this work?
There is only one Logrus. This is because, quite simply, the Sign of Chaos encompasses and includes every possible Sign of Chaos. It simultaniously exists in every possible universe.
There are, however, multiple Patterns. Corwin proved this quite conclusively by drawing his own, and creating a new universe. Creating? Not exactly. More likely, he merely added Order to an already existing one...
Trump seems to be more or less universal, though it takes different forms in different universes, and has only limited ability to pierce the barriers.
The Abyss is eventually the same Abyss, although you have to go pretty deep for this.
The wacky Sign of Chaos can be used just about everywhere. Logrus Mastery will work in practically any universe, which makes it a highly useful power for cross-universal travellers to possess.
The various Keepers of the Logrus, in fact, take advantage of their omnipresent sign to communicate with each other, turning the Logrus into a sort of trans-universal file sharing network. Part of their instability and eccenticity can very probably be put down to this.
It is not possible to use Logrus Mastery to reach another universe. You can call up the Logrus just fine once you get to your destination, but you'll have to use some other means to actually get there.
Rumors have it that one of the ways to cross universal barriers is to follow the outside curve of the Logrus, often said to be located in a cave or fen or black tower located in Chaos. However, it is hard to say how one would be able to control what universe you wound up in using this route.
Because of its nature, and the fact that multiple editions of it exist, Pattern is somewhat limited in cross-universal jaunts. While an initiate of the Pattern can still call up its image in her mind to serve as defense or for similar purposes, it grants no power over Shadow or Shadow movement. When you're on the turf of Pattern A, having the imprint of Pattern B isn't much good for influencing the Shadows cast by A.
White the Dworkins of the universes may be aware that multiple Patterns exist, its doubtful that they have any means - or even any interest - in communicating.
Pattern can be used to travel between 'nested' universes by traveling to the Primal Pattern of the 'lower' one, which resides in the universe cast by the 'upper' one. It cannot be used to reach between non-nested universes.
The exact nature of Trump is murky. Often it seems to vary from universe to universe, changing its nature and capabilities.
Despite this, Trumps are capable of cross-universal travel and communication -- under certain limited circumstances.
First of all, the person or place being Trumped in another universe MUST occupy the exact same spot in their universe that you occupy in yours. So if you want to trump Bleys-A in his apartments in Amber-A, you're going to need to make the trump call from the Amber apartments of your universe's Bleys. If you want to trump to another universe's Kashfa, you'll need to be in your own Kashfa.
Second, you need to have the cosmological imprint of the universe in question in order to make such a trump. After all, if you've never seen Bleys-A before, how are you going to make a trump of him? You'd probably just wind up trumping your own universe's Bleys.
There are exceptions to these rules, but they are very rare.
Trump artists can usually use their abilities in any universe. They may, however, run into difficulties in particularly oddball universes where the way Trump functions is dramatically different.
Magic is much the same from universe to universe, although, like Shadows, it seems to be stronger in some and weaker in others.
You couldn't cross universes with magic if you had a million years, a complete set of spikards, and every magical badass in existance helping you out.
Sooner or later, all Abysses open into THE Abyss.
However, you have to go pretty far down before this occurs. Exactly how far depends on the distance of the universes from each other. WR-Avalon and WR-Amber, for instance, are nested universes -- Corwin's Pattern's Uni and Dworkin's Pattern's Uni, respectively. You don't have to go down very far to get to where their Abysses merge. Gumi-Amber, otoh, doesnt merge with WR-Avalon's Abyss until very, very deep -- possibly deeper than even an Adavanced Abyss Master could survive.
Thus, the Abyss is useful for trave between adjactent universes, and less so for more distant ones.
Hard to say, given that the exact nature of the forces of Balance are poorly understood in most universes.
The power of the Outsiders may well be capable of crossing universes -- may even be what fills the splaces in between them. However, their influence seems to vary. In some universes, like TaMD, the danger of them breaking through is very real. In others, like WR, the Logrus and the Pattern laugh at them, and there's not a pseudopod or drop of ichor to be seen.