Absolutely. In fact, we recommend it. Think of direct mail marketers: they don't just look at what graphic works best. They consider the outer envelope, sales letter, inserts, and more. They look at fonts, colors, messaging, even the die-cutting of the pieces. A campaign is a multi-piece package. Every piece contributes to a successful outcome.
Web sessions work the same way. Consider subscribers to an online service. How often do visitors arrive directly on the subscription page and fill out the form without visiting other pages?
Not that often. Conversions rarely take place on a single page -- it's usually more like three to six page visits.
So think of a visitor's entire session. If you're offering a free product to every new subscriber, carry the offer from page to page: from landing pages to category or product pages to pricing pages to the final form pages.
In other words, you're creating an entire campaign like direct marketers do when they create a direct mail package.
There may be times you want to run a test on a single page -- if you simply want to find the best call to action for your home page, for example -- but more often, a multi-page test will get you more conversions and a better ROI much more quickly.