Strangely even though I'm right-handed I feel more comfortable juggling two in my left hand. I also bat and golf left-handed so sometimes I wonder if my parents forced right-handedness was on me.
I can, but I wouldn't describe having two two-ball capable hands as being half-way there. If forced to put a number on it, something like 20% is the best I could do.
I came in and out of 'actively juggling' through time, but I was at least 20 years with strong two in my off hand before four really started to do four for any real number of throws.
The perpetual issue was that the loops move in and out of sync, so the rhythm of responsibilities ends up with beat patterns that confuse my focus.
A good trick to practice 4 is to throw 4 throws in the middle of 3. So you juggle 3 balls then throw one to the same hand (the 4 throws) while holding a ball in the other for a beat (the 2 throws). You can put the 42 throws anywhere in the 3 pattern and if you do it as quickly as possible you get 423 which is an interesting pattern. 441 is good too - harder but helps with that sync problem.
The big step comes at 5. It took me nearly a year to master 5 balls with consistent practice. I eventually got reasonably good at 6 balls (juggled in sync, crossing over) but that's where I plateaued as far as numbers go.
There are lots of other things than numbers though. Non jugglers will have no idea how many balls you are juggling so you can impress with 3 balls. My favourite party trick was blindfold juggling. I used to be able to juggle 3 balls for about a minute like that.
It would be so useful to have two right hands. I'm curious whether you think getting over the hump helps with ambidexterity in general.
I had a class in junior high where we had time each day to mess around with juggling.
I got good at the balls and then got pretty good at the bowling pins things (not as heavy but same shape).
I could do 5 pins for a decent amount of time. It felt really good when I was in the groove of the flow, the arc, the spin and the sequence.
I tried 7 pins and was pretty excited to get a cycle or two successfully.
(I taught myself while procrastinating from exam revision, many years ago. I started with a large bag of oranges. Just enough of an incentive not to drop them, no real harm when they did hit the floor.)