You want the breather on the catch can as large or larger than the two hoses coming into the catch can, ie. two 20mm hoses you'd want at least two 20mm breathers or a 40mm breather.

a breather is for reducing crankcase pressure. to connect a breather to a breather means its not breathing at all (dipstick comes out etc.) the more power you make the more blowby you get the bigger breather system you need.
we are having probs with all the stroker motors we are doing as increasing the cubes increases the variations in crankcase volumes. ie we are fitting bigger breathers.
while on the topic of breathers dont forget the oil has to get back to the sump which is hard if all that air is coming up the oil drainback holes
we have been fitting oil drainback lines to rocker covers on some engines to help
stock engine? copy nissans setup they know more than us
dazza

I did this on my ca18. I was in a hurry..

Under boost the dipstick flies out, as does crankcase oil..all over the exhaust....creating a very nice fire in the engine bay.

Good thing it was on the Dyno..luckily didn't do much damage.

definitely put a catch can in, and run a hose from the can to the air intake box if you feel environmentally friendly.

I connect the 2 via a home made oil catch can, that automatically drain itself via the block breather tube.
In the top of the can I have an 8mm ventilation tube that opens up into the Air-filter tube.
I loosely filled the catch can with "Goldylocks" in an effort to prevent oil from splashing onto the breather-tube opening and getting sucked into the turbo too much.

This setup seem to work pretty damn well...

may also cause things like front/rear mains, and any other seal in the bottom end o leak oil.

so thats the worst of it?

blow by pressure will build in crankcase
causing it to burn oil.

would there be any effect or damage done by connecting the rocker breather to the crank case breather. im pretty sure there would be but just hoping im wrong