Could you ask you older one to help by tearing off toilet paper for his brother or something so he's involved?
tbh I'm waiting for both dtds to be ready - at 21mo they are playing at it. One says "poooooo!" and we all rush to the toilet for nothing to happen (until she jumps off, runs down the corridor and wees on the floor). Problem with using the toilet is that we don't have 2 next to each other and once dtd1 is on the toilet seat thingy, dtd2 wants in on the action as her sister is getting all the attention.
dd1 is now 5 and a lot better but she did have a phase of being lovely with them and then I'd see her kick them or push one over out the corner of my eye - we wrote a list of rules and stuck them up then toys were lost if she broke them, until she'd behaved well again. Initially she couldn't care less as she "had loads of toys anyway!" but now she dissloves into tears and tells us she's a rubbish sister and maybe it would have been better if she'd never been born! I dread to think what she'll say when hormones kick in! Having said that, she's fab now with them and incidents are very rare.
Cage? I'm being flippant, but actually I sometimes put my DD or the DTs into the playpen when I had to do something like that. We're behind you years wise, but my DD was/is untrustworthy with the boys, I got useless advice from the HV. Best thing that helped was a combo of lavishing DD with praise and trying to avoid any possible situations so I wouldn't get mad at her. Also, letting her have space that was her own that the DTs weren't allowed in. It made a massive difference when we moved house and she had a proper bedroom as opposed to a cupboard, she now goes off and slams the door and everyone is much happier!
My twins and their brother (dts are 2, brother is 4) need constant supervision. The older one picks on the twins when I'm not looking. One of the twins is Diego ready for toilet training, the other is less interested. How do I do it if I can't leave the other two unsupervised? We won't all fit in the toilet! I was hoping to avoid using a potty and even if I did, we're going to have to do the toilet bit at some point anyway. With DS1, we trained him before the twins could walk so I could leave them momentarily and sort him out. How do I do it?