Guild Wars 2

If I use, say, two spiders and one of them uses Poison Gas, once I switch to the other spider, will the CD of Poison Gas be carried to the second spider as well? Do they have totally separated skill CD?

Nope Spider A and Spider B have inner CD on they skill not same

-Spider A use poison gas and poison gas have CD.
-swap spider B
-Spider B can still use poison gas.

Same as Dog I try 2dog with passive mode Both dog can jump to knockdow enemy when command attack on 1st strike both.

-Dog A attack knockdown enemy.
-Swap Dog B.
-Command Dog B attack he start knockdown enemy.

I think this will need a more solid verification but I think they share the same internal cd.

Typically if you run bird, the first thing they will do is to cast swiftness. Even when you swap them halfway. However if you use 2 birds, the 2nd bird will take quite a while before using it again.