V.W.
How old is she?
If I remember right, tylenol can be given every 4 hours. For ibuprofen it is every 6-8 hours. So you would stagger them so that you are giving alternate medications. For example: if you gave tylenol at noon, then give ibuprofen at 1:30 pm, then tylenol again at 4pm, then you wouldn't be able to give the ibuprofen again until 7:30 pm (6-8 hrs AFTER 1:30 pm). Then tylenol again at 8 pm... But that is just an example. You would probably hold off on the tylenol a little longer with that dose until maybe 10 pm, just so she is getting meds in more evenly spaced intervals.
Personally, I never give medication for a fever until it has peaked. That way I know how high it goes (which can be indicative of what is causing it) or unless it goes up to 103. I figure it is doing what it is supposed to do by being elevated. They aren't really miserable until it goes up around 102 or more anyway. They might feel a little yucky- but a popsicle, a blanket and the TV/DVD remote and some doting from me usually helps with that.
But really, it is important how old they are. My kids are older (9 & 12), so I can be a little more relaxed about it. If your child is under age 2, then I would seriously keep a close watch and put in a call to your doctor's office (do they have a service?). Is there anything else going on with her? My son had a "run of the mill" fever last spring, and a cough. Nothing severe. Just a cold. But after he went to bed on Sunday night his temp went up and his breathing rate was really faster than it should have been with just a regular fever. Took him to the doc the next morning first thing-- double pneumonia. He'd had a cough all of 48 hours.
But yes, if the fever is high and not responding to medication, you can (generally speaking) alternate giving ibuprofen and acetaminephin (sp?). Just be sure you WRITE DOWN the time you gave each medication (Advil 5:00 pm; Tylenol 7 pm; etc). That way you won't misjudge how long between dosings, which could be dangerous.