Ubercart

It basically has two settings, which you create a combo of:

1. Select to apply the discount to:
   1. Multiple Products over or equal to a certain quantity
   2. Order Total over or under a certain amount
   3. Specific Product over or equal to a certain quantity
   4. Product Type over or equal to a certain quantity
   5. A Specific Taxonomy Term
   6. A Specific User Role
2. Apply the discount by subtracting:
   1. A specific $ or % from a Product Type over a certain quantity
   2. A Specific Amount from the order total by $ or %
   3. A specific $ or % from Multiple Products over a certain quantity
   4. A specific $ or % from a Selected Product's price
   5. A specific $ or % from a Product Type

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If you were trying to sell two identical products with the second at 50% you'd do the following:

1. Condition: Specific Product equal to a quantity of 2
2. Action: 25% from the order total

You can set this to repeat multiple times per cart and will do what you're looking for. Unfortunately it requires that identical items be purchased.

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If you were trying to sell any two products and have the second at 50% you'd probably not be able to get that exactly right. The closest is to give a 25% discount on the total order as above.

1. Condition: Multiple Products equal to a quantity of 2
2. Action: 25% from the order total

This has a loophole though - a customer can purchase a real expensive item and a real cheap item and basically get a 25% discount on the expensive item. Not a good idea if your prices vary too much.

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Another solution might be to offer a 50% discount on a second purchase of equal or lesser value (Buy anything at our shop and receive a 50% discount on any order of equal or lesser value on your next purchase) by either creating a per user discount under a specific amount (equal to or lower than the product purchased) or issuing a unique promo code after a purchase that's valid for and 1 purchase equal to or under a certain amount. Downside to this solution is not being able to offer the discount immediately. The upside is forcing a return customer.

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If you can think of another solution I'd be glad to hear it!

/* EDIT */
You might be able to duplicate your example by creating multiple discounts of the same kind with varying weights. However I'm still not sure how to apply the discount to the item of equal or lower value. I think this will have to be a feature request. Discounts_alt might be able to do it, but the shop I'm working on is a UC1 shop - so no go there.
/* END EDIT*/