Nicholas here, a Shopify Guru :)
Within the admin, there is no way to combine orders. However, we do have an excellent app called Order Manger, which will allow you to merge orders, among other order related controls not available natively within the admin :) I've included a link below :)
Order Manager App: https://apps.shopify.com/order-manager
Hope this help!
An update for this thread for SEO purposes lol...
Now that Shopify has the EDIT feature, refund 1 order (but set refund amount to $0 and do NOT notify customer). Then on the other order, Edit > Add the items from the other order (do NOT notify customer) > mark as paid then fulfill as normal. Works for me!