You could be me and in fact I was about to post something v similar tonight! I too have non-identical twin sons - a bit older than your boys, aged 8.
One (DS2 - born second) is plump, sedentary, loves reading and writing, well-behaved (teachers LOVE him) and has just had some v good school exam results.
The other (DS1) is wiry thin, actually slightly taller than twin, not interested in reading/ writing and whilst I used to think of him as the Mathematical one - he's just had a terrible Maths exam result and also did less well than his twin in English.
The more DS2 succeeds, the less DS1 tries and he already has v negative beliefs about himself. I'm v worried also, like you.
The weird thing is that DS1 did eveything first - opened eyes first, as a baby, talked first etc etc, is v v agile whereas DS2 is slightly dyspraxic. Unfortunately, he's not that good at sports either mainyl I think because he doesn't have the stamina of his peers and I haven't encouraged him enough.
I'm a single mum - from the start - and am much more like the geeky/ intellectual twin and realise that what I value is what DS2 is good at, whereas DS1 has many, may strengths that I don't have and should really admire more. I feel SO guilty!
Weird also is the fact that when the twins were about 3, DS1 was offered a place at a highly academic pre-prep. school but the same school rejected DS2 as being developmentally 'backward'! Fast-forward 5 or 6 yrs and now DS2 is at the Prep school with twins and doing extremely well, whilst supposedly v bright brother is 'just holding his own'.
The one thing that I think helps is that DS1 is much much better socially with his peers than DS2. In fact DS2 has many traits (but not a full diagnosis) of Asperger's and whilst he has a few good friends, equally 'geeky', is more or less unpopular with all the other 'ordinary' boys.
In the long run, DS1 may do better generally - like in interactions with people at work and in relationships etc, just because he's more accepted and good at communicating with peers, although a bit shy. DS2 may get better exam results but be more and more ostracised by his peers, particularly in adolescence and this makes me concerned for HIS self-esteem at that stage.
I know none of this is helping you but I just wanted to say that you're not alone with a twin situation like this. I'm pretty sure that the polarisation that takes place amongst siblings in all families is much more pronounced in twins. So eg one child is known as the sporty one - and therefore the other becomes so NOT sporty. One is great at English and the other refuses to read at all.
Despite being v aware of this polarisation and naturally worried about it, I still can't find a solution. It doesn't seem to matter what I SAY to DS1, he can still pick up in the atmosphere that DS2 is doing really well at school and he constantly says "DS2 is cleverer than me", which is not true.
In fact, in trying to decide what would be the right schools for them, I got their IQs tested. DS1 had a much higher score than his out-performing twin. However, DS2 was extremrly good at literacy stuff and also has a phenomenal memory, which can help hugely at this stage.
I think that as time goes on, DS1 will be able to use his better 'thinking outside the box' skills a lot more and succeed where DS2 may struggle, as DS2 has strengths in factual, logical learning.
I often wonder what life would have been like with only one of the twins and worry that lack of time/ input to each individually isn't helping...but I can only do my best.
So i don't know what to say to you really, except I suppose, like me, keep looking for the strengths in the smaller twin. One thing that helps me is that at school, my younger bro. was often in trouble, didn't do as well as myself and elder sister and was deemed less academic by our parents. But around age 16 and then again around age 20, he found the motivation within himself to strive and succeed and is now at the top of his profession and often in the media. So you can never tell how people will turn out.
Like you, I'd be v interesetd to hear from other parents with a similar situation with twins and if anyone can advise me too.