I'd alternate them using the bike one day and the treadmill the next day. Your body is extremely adaptive and even though you will start out losing and toning and building endurance, your body will adjust. You must also constantly tweak and change your routine. Do sprints, slow jogs, fast walks, spinning, inclines, etc. Every week try to change something in your workouts so that your body will never get "used to the same old exercise". Good luck!
"You never fail unless you fail to try." Enter the Haggis
"Sometimes you're the windshield, sometimes you're the bug; sometimes it all comes together baby, sometimes you're going to lose it all." Dire Straits
= 5 lbs. lost