If you join them from one to the other or block both on an A series you will first blow the front main seal out, then the rear then:

1. loose power
2. blow out dip stick
3. damage pistons
4. damage valve seals
5. blow out valve cover gasket
6. blow oil pan off


and so it goes, at WOT even a good condition engine will easily generate a massive amount of block pressure if it can not vent, as an idea a normal just broken in 1.6L 8valve basic motor will pump out 15-20L of air a minute at WOT, add 100 000km and it can go as high as 100-120L a minute and still be within specification.

Youll find breather systems become a lot more critical when you rev higher, when I used to run my motor to 8000rpm I had no problem with both breathers T'ing and going into a basic catch tank, as soon as i started to go closer to 9000rpm it would start to leak at the front main seal, I had to split them and run them seperate into a catch tank with a large 25mm ID filter, presto, problem solved

pic