Can I be a second driver on two separate policies?

You can be added to as many car insurance policies as you like as a matter of fact. The only thing that can stop you from doing so is finance.

Remember that as a named driver you do not get no claims bonus though and if you ever need to get your own insurance policy one day, you will start at zero no claims discount.

Yes, you can do that. Your brother will need to contact his car insurance company and ask them to add you as an additional driver on his policy though. His premiums may go up as a result, so bear that in mind.