Is it okay to workout twice a day?

Absolutely: As long as you fell post recovery from the workout and as long as you are maintaining an adequate healthy diet and fluid intake, there is no risk to working out twice a day. If you can keep up the routine, then you would achieve a great fitness level in no time. If your body gives up, it would give you signals such as pain, fatigue, illness - time to change your routine then

Yes: Ideally, you should aim for at least 30 minutes of moderate-intensity physical activity on at least five days of the week. http://patient.info/health/physical-activity-for-health

Workout: you need to be more specific with respect to what type of work yet you're doing. If you want a specific answer you need to be more specific with the question