I think you're wrong to try and completely avoid sibling rivalry. Simply because you won't succeed, however hard you try and in fact are more likely to make it worse.
I have two boys two years apart. They both take part in the same 3 sports, football, swimming and karate. Obviously, ds1 (6) is noticibly more advanced than ds2 (nearly 4) in all of them.
I give them the same praise after the session, talk about their own achievements within it (eg, swimming - 'Well done ds1, you did two lengths without stopping!' and 'Amazing ds2, you managed to kick for a whole width!')
I encourage ds1 to always help his little brother and be a 'graceful' competitor (there is a fantastic episode of Sophia The First which we've seen recently about winning, losing and trying your best which has actually been a huge help with this attitude!)
If ds2 ever groans about ds1 doing more/better (ie ds1 is now in the deep end at swimming and he's still in the shallow end), I just remind him that ds1 is two years older and that when he is that age he'll probably be in the deep end too. We move on, it's not an issue.
I think if I tried to make them do separate sports it would cause resentment from both of them as time goes on.
I was in the situation as a child that I was significantly more academic than my sister. I constantly felt guilty and almost ashamed at my exam results and achievements and got so used to having to play them down that I stopped mentioning anything, even really big stuff like being accepted into the Year 11 debates team when I was in Year 9.
It shouldn't be like that IMO. EVERY childs achievements should be celebrated and compared to their own 'personal best' within that area and not their siblings.