Since everyone is advising you to buy more stuff, here's what I would do:

Don't grow for awhile. Just keep buying weed, but buy a little less than normal to save money. Think of other ways you could spend less. Sell old crap you don't use anymore. Then, after a few months of this hardship you will have enough money to set up your grow the right way. First choice: have a dedicated flower area and a different veg area, with two lights and ballasts. That way you can harvest every 60 days or so instead of 120. Second choice: Buy a switchable ballast and have one grow area. Third choice: Do what you are trying.

Trust me I have been where you are and tried all the above choices. With all the time and money I spent fiddling about with choice 2 and 3, I could have easily paid for choice 1.

And don't forget, and this is very important: Spend some good money on ventalation. Cool tubes are great (search this if you don't know about it). Believe me you will be sorry if you don't properly ventilate. If you find out too late like I did that you don't have enough air flow, you'll have to tear everything apart and start over.

Good luck.