D.K.
First I don't have twins, howver have two kids that can feed off each other's bad behavior. Do not punish the one that did nothing, that will blow up in your face. If they get into trouble for doing nothing, then what is the point of not just doing it anyway? Make sense?
Just because they don't go in their diaper does not mean they are potty trained. Potty trained is knowing the urge, going to the bathroom, waking up dry and being able to hold it until they are in the bathroom! I say they aren't there yet. If you have to go back to pullups and let them know until they are ready to be big boys they are back in them, period. Boys don't typically train easily or early so being close to three it isn't uncommon they aren't ready.
If you don't want to do pullups you need to really start working hard with them. You need to set a timer, every 40 minutes and hand hold them to the bathroom. Not only is what they doing gross it is unsanitary on so many levels, what is to keep them from pooping and playing in it? They have no consequences that matter and you need to take them to the potty every half hour throughout the day, no matter how much of a pain it is for you and teach them to hold it and go to the bathroom. I would go hard core on this and really drive the point home to them it is UNACCEPTABLE to urinate or poop on the carpet.
Especially if they know how to go to the potty and choosing not to out of being lazy.
Also for boys try a chart for behaviors, they like visuals and striving for something. So, every day they go to the potty willingly, without issue and do not pee on the carpet a happy face, for every infraction take it away or a frowny. Tell them they have to earn six smiley faces in a row and then maybe an ice cream or a reward that appeals to them. It is training and enforcing positive behavior. You don't have to do it forever, however with a four 1/2 year old boy I do know that he likes seeing his progress, can relfect back on a bad day with a visual!
Some carept cleaners can be toxic too and I see you have a 20 mos old.
If they both do the deed, they both have consequences. Find what matters, if time outs don't cut i, then try taking toys away for a day, losing increments off bedtime, etc. If they are running the show now you are headed to trouble later.