Which Marathon do you have? The older ones are actually taller than the newer ones, even though they have lower weight limits. If you have the older one, you should be fine RFing even a semi-tall kid (maybe not an off-the-charts kid) to at least 2. Even the newer ones will last most kids to age 2, but not really tall ones.
The biggest concern would actually be leg room. This is not a safety issue, and for most kids it isn't a comfort issue either (though there are exceptions). It's just a parental issue. :) Parents don't like the way it "looks" to see kids' legs bent or criss-corss. For some reason it's hard to convince ourselves that the kid is comfortable even when the kid is happy and shows no sign of distress. And if the kid doesn't like being strapped down (which most kids go through that stage a few times in the toddler years, no matter whether they're FF or RF) we convince ourselves it's because they're uncomfortable for lack of leg room. In the vast majority of cases, this is just our parental imagination.
Anyway, if you are concerned about leg room, the Britax convertibles are almost the worst (I think Recaro is worse; it's actually so bad a lot of kids really do get uncomfortable). There are other car seats out there with much better leg room. The Diono Radians and The First Years True Fit come to mind.
One thing to consider is when your Marathon is going to expire. Maybe it makes more sense to leave the older one in it until it expires, then switch him to a booster, and get the younger kid a seat with more RF leg room which will last until that child is booster age and will give you the flexibility to RF the younger one longer than age two should you choose to do so. Or maybe it makes more sense to pass the Marathon down and get the older kid a FF seat, then when the Marathon expires play the hand-me-down game all over again and get the older one a booster. Or maybe the Marathon expires in 6 months and both kids need new seats anyway.