Yes, it is possible to have two ethernet ports on a laptop. While there is no laptop that I am aware of that comes with two, you can still add a second device. Ethernet adapters come in both PC Card and USB varieties, so assuming you have a free USB port or PC Card slot you can simply plug one in. In this configuration, you can use the onboard network adapter, and the USB/PC Card adapter at the same time.
Ethernet and modem.
The Mac Pro has two Ethernet ports so that two Ethernet cables can be connected to the computer. The cables may be connecting a local network or peripheral devices such as printers or cameras that require an Ethernet connection.
USB ports
The difference between an ethernet bridge and a switch is that the bridge only has two ports, while the switch has multiple ports. When there is a lot of traffic going through the ports, the bridge traffic may get tied up or crash often because there are only 2 ports for information to travel back and forth.
In the case of ethernet, you need a "crossover" ethernet cable connected between two regular ports or a regular ethernet cable connected to an "uplink port" on one switch and a regular port on the other. Uplink ports are sometimes called "MDI" ports. Both types of cable are available at most computer and office supply stores.
You have two basic options. 1.You can connect your laptop to your cable, fibre optic, or DSL modem or Ethernet port using an Ethernet cord. 2. You can connect your cable, fibre optic, or DSL modem or Ethernet port to a wireless router using a Ethernet cable. You then connect your laptop using wireless to your wireless router.
Most modern laptops lack any sort of "legacy" ports such as serial ports, let alone them having two. Some older laptops did have multiple serial ports, but there's little point in recommending one because there's no guarantee you could find one. There are options for adding serial ports to a laptop, such as a USB to serial adapter, or a PC card with additional serial ports.
Your laptop, and probably your PC will probably not have 2 Ethernet ports to connect two Ethernet cables. It is possible to use both WiFi and your Ethernet port. It is also possible to add an additional Ethernet card to a PC or add a USB or PC-card based Ethernet interface. If you did have a machine with two Ethernet ports or wanted to use both an Ethernet port and a WiFi connection, there is a routing problem. The system needs to know when to send (route) something to the internet and when to send it to the other computer (PC or notebook). You would need to manually assign the default route (for the internet connection) and with a different network id, assign network IP addresses to the connections between the notebook and the PC. How this is done depends on the systems you are using. It is different for Microsoft Windows and Linux implementations and may even vary depending on the version of Windows. A more practical method is to connect both machines to your internet connection, your router. They will be on the same network. They will both be able to access the internet. They will both also be able to communicate with each other through the router. Learn how to, or find someone to, configure file sharing on both machines and establish which file and/or directories should be visible to the other machine or machines on your network. If you are using a Unix or Linux version you can still share files by configuring a program called Samba. You will need do learn how to do it or find someone who already knows how.
USB and PS2
Yes it actually has two USB ports that are available on either side of the bridge.
No, why would you? I suggest just buying a wireless PCI card for the two desktops. It would make life much more easier. You can, what you need is a wireless router... this will allow you to have approximatly 4 ethernet ports and wireless access to your network, but look in your manual to make sure you encrypt the data so as to protect from wireless predators...
The laptop has 2 usb ports on each side. The ports are approximately 1/8" apart.