DT1 and DT2 are same ability. One has just started grammar, one comp. DT1 did not do as well as expected and it's been a hard year for them 
They both scored 120 in Maths SATs. It's their favourite subject (their other SATs scored similarly highly but they love maths in particular).
I do wonder if there will be a gap by KS4 and whether that will widen a little each year (based on comparitive GCSE results)
DS2's grammar have chance to study further Maths (i think) and additional GCSEs, where DT1's school offers 9 GCSEs but no further maths.
I dont know much about the qualification and whether it will still be running when they take GCSEs, but is there anything I can do to keep things as 'equal' as possible?
The obvious answer would be same school. However DS1 had his heart set on grammar and it was only his push and confidence that separated the pair on the day.
I feel guilty that DT2 may get more opportunities and progress further when they are equally able. The chance to take more GCSEs and a broader mix/advanced maths.