You won both lenses, you should pay for both lenses and sell one on.
Why should the seller have to be inconvenienced because of your buying tactics. You should wait until the first auction has finished, then bid on the next item if you don't win. Bidding on more than one, than choosing which to buy if you happen to win two or more is not how you should deal on ebay.
I sell a lot of stuff on ebay and, fortunately, it is rare for a winner to default on the deal. When it happens, it costs me in time and potential loss of a high bid, as I sell a number of linked items, so bidders tend to buy several different things at once. An unsold item, later relisted on it's own, will usually sell for less.
Like a land auction, you are legally committed to following through on the purchase. Ultimately though, it is unlikely that you would be taken to court over it, so it really comes down to whether you are an honourable person or not and make good on your commitment.
Sorry if this sounds harsh, and seems a bit unwelcoming to the site, but the fact that you are even thinking about doing this really annoys me.