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It's not a good idea to turn a product into a free product to support another one unless that other product is something like Google's Search Engine.

Even then, you could argue it's better to build multiple streams of income and not depend on one product.

An offer of Buy 1 product and get the other free - that's fine. But giving away inbox cleaner doesn't make sense unless you get a really high conversion rate for Tweeting Machine.

You have also asked a completely different question in your blog post. To sell more tweeing machine subscriptions - perhaps a longer trial (1 or 2 months instead of 10 days) or a lite version would make sense.