Do you usually use the same vehicle on the way somewhere as on the way back? If so, then you could always leave the infant seats in the vehicle as the twins get heavier - you mentioned that you sling them, so I'm guessing this might be something you're doing already?
Pepse is right that the SS1 would be heavy to cart with the twins in it at their weight, so either way you'd be most likely looking at carrying the kid and carseat separate.
If most installs are going to be done at home, then the need for pool noodles likely isn't a big deal - I'd actually just use a rolled towel, probably quicker to adjust for the variation in vehicles seats that you encounter and would be easier to move around. If you had to change between different #'s of noodles, it could get to be quite a pain.
I wouldn't go with the Radian for the same reasons others have mentioned. With not having the same car, it would be unpredictable as to whether or not the seats would install every time, which is a stress you wouldn't want. They also would be a tight fit rf'ing.
I'm inclined to agree with the recommendation for scenera's. The EFTA and the Marathon are both fairly wide seats, and it would be unlikely that you'd be able to fit 2 next to each other and an adult outboard in many if not most vehicles. While I don't think the $70 price tag on the Scenera makes it as good a value here as it is in the US, it does fit the need for narrow and lightweight - making it easy to travel with. 2 Scenera's would also be cheaper than 2 SS1's, and either way you'd end up with having to buy new seats for the twins before they were ready for a booster. Overall you'd get more use from a Scenera.
With a little bit of practice I think you'd find the Scenera's relatively easy to install too. Aside from needing a towel or pool noodle in most vehicles, they're not too hard to install most of the time.
Ideas for if you go with a seat that needs help with recline, would be an extra backpack to store the towels or pool noodles in if you do end up having to carry the seats with you between your arrival and departure. Then it would all be contained easily enough. :thumbsup:
If you did decide to go with a Britax convertible or the Radian seat, I don't see it as being feasible to rf tether with moving the seat around so much, so I don't think in this particular instance that being able to rf tether a seat is much of a benefit.
I'd go less expensive seeing as how you'll have to buy new seats when the twins are older unless you end up with a MA, EFTA, or Radian. And I don't think any of those options are particularly good for your situation - due to width for the first 2, and tricky installation/front to back space for the radian.
If you call your local Canadian tires, some of them still have the 45lbs Radian for $129. Mine was able to tell me which stores around were selling it for $129 where theirs was $199 but they didn't have any in stock.
The CDN tire Radians had 48lb weight limits. :thumbsup:
Awesome that your CT was able to tell you where to get a discounted seat, thanks for sharing!