I think you can do them separately .
I think the consensus that they are not ready has come from your previous posts, where it seemed neither were progressing and both were reluctant.
It doesn't seem fair to hold one back because the other isn't ready.
It might be confusing and send mixed messages to tell him to go ahead and use his nappy when he actually wants to use the toilet.
Him holding his wee/poo in, and saying 'no not nappies' is as a clear sign that he is ready as you're ever going to get, I think.
I'd try and stray not too far from a toilet/bush, put underpants on him and take a spare pair of clothes including socks when you go out. I found it helpful, to go out as little as possible when they were at the early stages of potty/toilet training.
The advantages of doing this separately, as I see them are..
It'll probably be easier to tackle this one at a time.
It'll be sending out an important message that it's OK for them to be different, and to be at different stages.
One being keen to progress is likely to spur your other son on, and as long as it isn't seen as a competition, I think this is a positive, too.
Even if this doesn't happen, you'll be halving the cost you spend on nappies, which with twins costs a fortune.
Good luck with it all, I bet they'll have cracked it before you know it. 