My boys have followed this pattern. Alex was very trying at 18 months, with his massive sleep regression and hour-long tantrums. His disequilibrium phase lasted from 15-21 months. Nate was very (OMG VERY!) trying at 2.5 with his complete defiance and boundary testing. In his disequilibrium phase from 2.25-2.75, I was fairly convinced we could try to give him away and no one would take him. And while their twin was also going through a disequilibrium phase, there was always one who took it to the next level! [random Rob Huebel reference]
Everything (and I do mean everything) I've read says 3.5 is the disequilibrium phase to end all disequilibrium phases. Even Ames' 3 year old book says to GET READY. And guess what? The boys turn 3.5 next month and this phase is not really that bad. I'd take this phase over 1.5 or 2.5 any day.
When the boys were 2, we had to learn all about our discipline style. When they were 2.5, I felt like I spent more time being a referee than I did being a mother. This behavior carried over into school when they had a THREE MONTH biting streak where they bit each other on a regular basis. But all this insanity and rule testing was a good thing because Jon and I developed a solid set of House Rules. We learned what we can let slide. We learned how each boy responded to discipline, and how we reacted in various situations. We learned our own emotional limits.
Out of this, I turned into a mean mom. HA! I say mean, but I am not mean, I am strict. We have rules for a reason and we expect them to be followed. We don't have rules for no reason and we don't say "Because I said so." The boys get two chances to correct their behavior... to learn... and if they do not, they go to timeout in their room. Some of the "meaner" rules are that you must wash your hands, you must brush your teeth, you must wear a bike helmet. There have been many tears over these rules but they are non-negotiable.
So... as the age 3 has come upon us, we have a solid discipline plan that does not involve emotion. When I get overly emotional about their behavior, it's when I am not following my rules of discipline. Do they test us? HELL YES but it is nothing compared to what we went through when they were 2. Do they throw tantrums? HELL YES and they go straight up to timeout until they are done. Do they act terrible and trying? HELL YES but they know the rules. And the difference is, at 3 they are old enough to understand the rules. And at this point, there is very little they can show us that we haven't already seen.
Then you add in the most delightful development EVER... the constant playing together. They get into elaborate games with one another for extended periods of time. Jon and I went to a playground and actually got to sit for an hour chatting with each other. No getting up to make sure they wouldn't fall off something. No one running off the playground. No one pushing each other off some rocks. No one smacking the other on the head with a shovel. No diaper changing. Literally an hour with no interruptions. Only another twin parent would get how HUGE this is.
Sometimes I am at a loss on what to do with myself because the boys will be happily playing in the playroom together. After these crazy toddler years, I am so used to spending my time on alert in case something happens. I'm so used to getting food, refilling drinks, changing diapers, stopping fights, and running around that sometimes I can't believe how much calmer everything is now. I am quickly getting used to this newfound freedom.
I will make this official. Age 3 is when we are finally reaping the benefits of having twins. Age 3 is where I finally feel confident in my parenting. Age 3 is where all the work we have put into managing two kids is coming to harvest. Age 3 is my favorite age yet.