How low can the straps be when RFing?

[curiouscanadian]

I will shortly be picking up my 1.5yo nephew after work daily, in addition to my 2.5yo. 32 lb DS is currently RFing in a Radian, and we have on hand a Britax Marathon and some other convertible carseat my step-sis has (haven't seen it so don't know what it is).

Since there is no way I can figure to have both boys RFing and still be able to drive our '05 Civic can I have the Radian straps set low enough to work for my nephew and then use them in that same (super low) position for DS the rest of the time? I would then install the Marathon FF behind the driver for DS to be in only for the short drive home.

(I should maybe mention that DN is over 30lbs (he's a big boy) so can't RF in our Marathon, and we've tried having the Radian RF in the centre before but even at 30deg, it pushes the driver's seat too far forward.)

[an_aurora]

I was just going to say "I assume you are in Canada, from the 30 lb limit Britax?" and then I saw your SN :p  So, to answer your question, it does not matter how far below the shoulders the straps are, but sometimes if they are WAY below the shoulder, they can slip off a bit.  

[butterfly_mommy]

I also find it is not a huge problem to change the strap height on the radian when it is rear facing. You can do it with the seat still installed.