If they pass the test, they'll cope with the work. so just let the test decide.
And remember, although some children are brilliant at everything, most have areas they're good at, and areas they aren't, so there is a range of abilities in any given subject, even in a grammar school!
It also doesn't really matter about sending children to different schools. That's you projecting, sorry to say. The important thing is to get your own thinking sorted on that, and to not let it colour how you respond to your children's particular strengths and weaknesses.
Being academically strong isn't the be all and end all. Neither is it a bad thing. They'll both have differences, positive and negative; you have to celebrate what they are as individuals.
With regard to the entrance exam, my opinion is now that, if the exam is NVR and VR, all you need is to make sure they are familiar with the format (and that means buying some commercially available papers in the same format they'll sit) and getting them to do them. Good literacy (lots of reading) and excellent times tables.
If there's a maths paper - make absolutely sure they've covered the National Curriculum for yr 6 by the time they sit the test. You can't count on the school having done that.
If English paper: vocab., spelling, tenses.