Oh, there are lots of options here, all well under $500. You really don't need much for summer bikinga sleep setup rated to the mid-30s should be more than adequate. One excellent choice is the Feathered Friends Sandpiper ($275 with nylon shell; www.featheredfriends.com). It's a down-filled, semi-rectangular bag with a zipper that extends around the foot, so it can be unzipped, laid out flat, and used as a blanket for two people. Feathered Friends makes an accessory piece called the Toucan ($60 in brushed polyester) that zips to the Sandpiper so you have a sheet beneath you. Then set the whole thing atop two sleeping pads and climb in. As a stand-alone bag the Sandpiper is rated to 30 degrees. A little hard to say how that would translate as a double bag, but I believe it would still be fine in that range. After all, the insulation is crushed beneath you when it's used as a single bag, anyway, so doesn't do you much good. If you're worried about sleeping cold, Feathered Friends makes a bag with a similar design called the Penguin ($300) that's rated to 20 degrees.
Another popular choice is to buy two Cat's Meow bags from The North Face ($169; www.thenorthface.com) with opposing left and right zippers so you can zip the pair together. That would be a very warm setupthe synthetic-fill Cat's Meow is rated to 20 degreesbut is also somewhat bulky and heavy (about six pounds total). For that matter, just about any maker's bags are mateable if purchased with opposite zipper configurations. And for a setup similar to what you can get from Feathered Friends, take a look at the Marmot Wasatch ($199; www.marmot.com), a 25-degree, semi-rectangular bag that mates with a second Wasatch or unzips and lies flat as a comforter.
For any double-bag system, get yourselves a Couple Kit from Cascade Designs ($7; www.cascadedesigns.com), a set of paired loops that holds two pads together, so that you don't end up with a big gap on the ground between the two of you.