Have to Blend Forehead Botox Treatment with Glabella Treatment?

You can get the forehead and the glab areas done separately but it is less expensive if you do them together. Cost is usually $300-350 for 1 area and $500-600 for 2 areas whether it is botox or dysport and lasts about 3-4 months.

The key to good results with Botox to the forehead and glabellar region is recognizing that the two muscle groups pull your eyebrows in opposing directions.
Treating your forehead (frontalis) muscles in isolation can alter the position (lower them) of your eyebrows. If your brows are in good position (no ptosis), try a very light Botox treatment to the upper forehead (no more than 6-8 units) and observe its affect on your eyebrow position over the ensuing weeks.
Your best bet with Botox is to have the glabella, frontalis and upper/lateral orbicularis muscles treat in concert. Such a treatment plan might even elevate your brow position, which is nice.
Seek out a clinic staffed by a board certified plastic surgeon for your Botox needs. A good Botox injector will thoroughly examine your face at rest and during movement and design a treatment plan that is tailored to your individual musclular anatomy and brow position.

If you want to try to treat the forehead only you need to be with a very experienced injector.  Treating the forehead alone can give you a very heavy brow and flat eyebrows even with some of the most experienced board certified physicians.  Treating the central area between the eyes (the glabella) is what gives you that eyebrow lift.  My advise is to put at least a little in the glabella area, but that being said you can try forehead only and add the glabella in 1-2 weeks if you feel flat or heavy in your eyebrows. 

You can certainly treat the forehead lines by themselves and not treat the frown lines. You may still want to put a small dose of Botox into your frown lines as a preventative treatment to prevent needing higher doses there in the future. More and more, we are using Botox not just as a treatment, but as a prevention so that lines/wrinkles don't "etch in" which is much harder to treat.

Your forehead can be treated on its own, but make sure you see an experienced injector so you do not develop lowering of your eyebrows.  When your frown lines are ready again for treatment, your forehead should be reassessed as well.