I have a Saris Bones 3, although I haven't used it that much. My car's a Skoda Roomster, which has a near vertical tailgate. With two solos and a kiddy bike, the rack tends to pull away from the back window unless the straps are REALLY tight due to the geometry of the thing. With a sloping rear window this issue may not be so bad, but then the bikes are going to bang into each other.
I'd be very wary of adding any more weight than 3 solos - although with the wheels off the tandem that might not be so bad.
If you can afford it, a towball mounted rack would be a much better option in my opinion.