The only thing I can think of is to use an image for the Slashed Price and hard code it in. The price of a product weather Discounted or Retail is brought in by a single variable and so it will not be possible to display two prices for a single product.
There is some thing else you can do... you can use a custom Property for the second price and to get the strike through effect you can insert the following code in the Product Layout Template you are using for the specific product...
<font color="red" style="text-decoration: line-through;">CUSTOMVAR:OLDPRICE</font>
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