First, bear in mind it's not necessary to set prices in different currencies in order to accept payments from international buyers, as the payment processor or card issuer performs currency conversion automatically. E.g., if your prices are set in GBP, then a US buyer could simply pay you in GBP, and their payment amount would show up on their account statement converted to USD.
If you would rather create duplicate products priced in different currencies, you would need to manage those as separate, independent products. Also, since a buyer's cart can only contain items priced in the same currency -- clicking Add to Cart for an item priced in a different currency would reset the cart and jettison any items added previously -- to prevent buyer confusion you would want to either use our Buy Now buttons for instant checkout of a single product at a time, or otherwise use separate sites or a strict subdivision of your site to prevent buyers encountering purchase buttons for both currencies.
