Both have the exact same active ingrediant which is T4 synthetic thyroid. The difference is they may use different buffers or fillers which may or may not affect things such as asorbtion or allergies or sensitivites in certain people.
In general it should be no problem at all mixing brand name versus generic T4 meds.
Hi ando... Yes, it's okay to do that; I've done that and didn't have any problems with it. Some people will tell you that Synthroid is so much better than generic, but I actually did better on generics. In my opinion, you should be fine.