nodejs accessing sessionStore in another source file (to prevent duplicate login)

I'm trying to implement 'duplicate-login prevention' routine in node.js what I intended is

1) create mysql-table UserSession(userid, sessionID)

2) for each valid login request, check sessionID for given userid

3-1) if it's NULL, proceed 4)

3-2) if it's not null(=session already exists), end that session

4) give new session

but I have some trouble in 3-2) these are my code(I'm using express4)

app.js

var routes = require('/routes/index');
var session = require('express-session');
var sessionStore = new session.MemoryStore();
var app = express();
//...
app.use(session({
    secret: "secret key",
    resave: false,
    saveUninitialized: true,
    store: sessionStore,
    cookie: { maxAge: 10000*60*60 }
    }));
//...
app.use('/', routes);

index.js

var router = require('express').router;
var accountUnit = require('./accountUnit');
//...
router.use('/account', accountUnit);
//...
module.exports = router;

accountUnit.js

var router = require('express').router;
//...
// login/logout function
//...
module.exports = router;

As I think, I have to access to sessionStore(in app.js) in accountUnit.js to implement 3-2). But it seems unrealistic because requiring is one-directional. It seems I can only access to req.session rather than the whole session list(sessionStore)..

Q1. Is there any ways to use sessionStore(in app.js ) in accountUnit.js ?

Q2. Is my idea about preventing duplicate login is right?