"Can I just teach both of them at once?" If you have two children in similar year groups, the temptation is real. Here's the honest assessment.
When it works
If siblings are temperamentally relaxed about competition. If they're at similar levels. If parental attention can still be split during review.
When it doesn't
If one is significantly stronger and the comparison becomes constant. If one is preparing for a real 11+ and the other isn't. If the dynamic between them is already competitive. Practice now independently if you're unsure.
A workable middle path
Joint sessions for reading aloud and vocabulary games (low-stakes, sociable). Separate sessions for mock papers and reviewed scores. That mix preserves the benefits of joint work without the comparison pain.