I have a built in pci wifi card and a usb wifi card. If I have both of them connected to the same router, will windows use both of them simultaneously or prefer one over the other? If I have them connected to separate networks what happens then? (I cant test this out as there is only one router I have access too (my own)). Will my speed go up in either case? and finally... the same questions as above except on linux and mac.
By default on any OS, it will basically only use a single interface for any communication initiated by your system.
The interface that is used is set by the route metric.
Incoming connections from the networks local to the interfaces that target a specific interface address will be handled by the interface that was addressed.
There are methods to combine capacity, but the are mostly focused on server setups. The require advanced routing or bonding.
Multiple network interfaces may be used simultaneously to provide access to different networks with various resources: home, public, guest, corporate etc.
If you have more than 1 interface connected to the same router, then the first network connection specified in "Adapters & Bindings" list will be used. Bindings determine the order in which network connections are accessed by network services.
Bindings can be configured in Network&Sharing Center in Advanced Options. Bindings settings
Network throughput can be increased if link aggregation is used on a host.