How do you properly split up casperjs tests with a shared browser instance

This will not be easily possible and potentially dangerous. Every action that you do, would need to be reversed to not break the other tests. If you later decide that writing tests in a modular manner is a good thing, you will have a headache writing your tests.

PRE.js will be your start script which you modify to execute your tests in between. In the following fully working example you see how you can schedule multiple test cases for one execution of casper. This is bad, because the canvas test case depends on the proper back execution of the link test case.

casper.start('http://example.com');
casper.then(function() {
    this.test.begin("link", function(test){
        var url = casper.getCurrentUrl();
        test.assertExists("a");
        casper.click("a");
        casper.then(function(){
            test.assert(this.getCurrentUrl() !== url);
            this.back(); // this is bad
            test.done();
        });
    });
    this.test.begin("canvas", function(test){
        test.assertNotExists("canvas");
        test.done();
    });
});
casper.run();

Of course you can open the root again for the new test case, but then you have the same problem as with your initial code.

var url = 'http://example.com';
casper.start();
casper.thenOpen(url, function() {
    this.test.begin("link", function(test){
        var url = casper.getCurrentUrl();
        test.assertExists("a");
        casper.click("a");
        casper.then(function(){
            test.assert(this.getCurrentUrl() !== url);
            test.done();
        });
    });
});
casper.thenOpen(url, function() {
    this.test.begin("canvas", function(test){
        test.assertNotExists("canvas");
        test.done();
    });
});
casper.run();

Now the test cases don't depend on each other, but you also load the page multiple times.

---

If you need some initial actions for every test case then the PRE.js is not the right place for that.

Create include.js and put the following code there:

function login(suite, username, password){
    username = username || "defaultUsername";
    password = password || "defaultPassword";
    casper.test.begin('Test login', function suite(test) {
        var done = false;
        // event handlers

        casper.start('http://localhost:' + port, function() {
            // login if the session expired or it is the first run
            if (!loggedIn) {
                // login
            }
            // wait
        });

        casper.then(function(){
            suite.call(casper, test);
        });

        casper.run(function() {
            test.done();
        });
    });
}

Then you can run it as casperjs test casper_tests --includes=include.js with test files like

login(function(test){
    this.click("#something");
    this.waitForSelector(".somethingChanged");
    this.then(function(){
        test.assertExists(".somethingElseAlsoHappened");
    });
});

Of course you can have different login functions (with different names) or more lightweight ones.

---

Building on the previous snippets, you can make a start script and load the test files yourself. Then you have all the flexibility you need to do this.

include.js:

function login(testScript, username, password, next){
    // event handlers

    casper.start('http://localhost:' + port, function() {
        // login if the session expired or it is the first run
        // waiting
    });

    testScript.forEach(function(case){
        casper.thenOpen(case.name, function(){
            this.test.begin(function suite(test){
                case.func.call(casper, test);
                casper.then(function(){
                    test.done();
                });
            });
        });
    });

    casper.run(next);
}

start.js:

// pass the test file folder to the script and read it with sys.args
// fs.list(path) all files in that path and iterate over them
var filesContents = files.map(function(filename){
    return require(filename).testcases;
});
var end = null;
// stack the test cases into the `run` callback of the previous execution
filesContents.forEach(function(case){
    var newEnd = end;
    var newFunc = function(){ login(case, u, p, newEnd) };
    end = newFunc;
});
end(); // run the stack in reverse

each test file would look like this:

exports.testcases = [
    {
        name: "sometest",
        func: function(test){
            test.assert(true)
            this.echo(this.getCurrenturl());
        }
    },
    {
        name: "sometest2",
        func: function(test){
            test.assert(true)
            this.echo(this.getCurrenturl());
        }
    },
];

This is just a suggestion.