Tapastic doesn't do double-page spreads due to their preference for a vertical, mobile-friendly reading experience. You can either upload it as it is, at 940px wide, and put up with the loss of detail from the small size, or you can mess around with the page a bit, cutting it up so it better fits the format here.
If you plan on using Tapastic as the primary home of your comic, I would recommend avoiding double-page spreads altogether. If you really want them for a print edition, you might want to get a little creative when you upload them here.