I've tossed around buying a sit-n-stand, and ended up with a regular double for this exact reason... Still wish I bought the sit-n-stand!
I've thought of, but haven't tried yet... making a Sling (think hammock) for the backseat/platform. Use 2 belts or small straps used for tying down items and sew a continual loop of fabric (think over sized t-shirt sleeve). The fabric loop would need to be large enough to go from the top of the bars for the back rider to the front portion of the rear bench. Put 1 belt through the fabric loop and attach to the stroller just above the back riders "handles" so it can't slide down, and attach the other belt to the opposite end of the fabric loop in front (between the front and back seats) so it can't slide back. The kid would sit on the fabric and lean back on it for support. Slide the bench to the rear of the stroller, put kids feet into the basket area...
Does any of that make sense??? It works... in my head, But like I said... I haven't actually tried it.