The first question is: What are the condos worth? As is suggested below, it's possible (in fact probable) that the listing price reflects current market conditions. I know of condos that, pre-bubble burst, sold for $300,000. Now they're listed at $145,000 because that's an accurate reflection of market conditions. So, determine their real value.
Now, I tend to agree with you that you can ask for a discount if you're buying both. Although Lori's correct that the transaction costs on the two units will be double that for one, you are offering the seller something of value: The opportunity to sell both condos at once, rather than selling one now and waiting 60 days, 90 days, or more for the possibility of another sale. As the saying goes, "A bird in the hand is worth two in the bush."
This is a situation where you really have to strategize with your Realtor. And you have to attempt to determine the seller's motivation, as well as how much equity he/she has in the property. As Bay observes, if you offer so little that it won't even pay off the mortgage, your offer won't be accepted.
However, don't worry about insulting the seller. Worry about making an offer that works for you.
As for the point that properties sell at, say, 95% of their listing price--don't worry about that, either. For instance, suppose the condo starts off priced at $100,000. After 30 days, it doesn't sell and the owner reduces the price to $95,000. After another 60 days it still hasn't sold so the owner reduces the price to $92,000. Then it sells for 95% of $92,000...or $87,400. But, notice, that's only 87% of the original listing price. And let's take this a step further. Suppose there was a 3% seller concession (to allow for a no-money down FHA loan) or a 3% contribution to Nehemiah. Seller concessions don't show up as a price reduction. Net to seller before any expenses and commissions would be $84,778...or 15% down from the original listing price. Yet the statistics would show a sale at 95% of the listing price.
So: Figure out what the condos are really worth in today's market. Factor in anything you know about the seller's motivation and/or the amount owed. Reduce that number a bit for negotiations and for the benefit you're offering by buying both at once.
I have no idea what that number might be. It might just be 5% off from the current asking price. Or it might be 20% or more. But that's not the point. The point is: Do the calculations. Make sure you know the values. Then make an offer that works for you.
Hope that helps.