If I connect 2 3.0 Ghz PCs together, will I get a 6.0 Ghz machine?

No and no. Some dial-up ISPs allow you to use several phone lines to increase the speed, but they have to support splitting requests, and unless your ISP is some really special one, you're not going to be able to do it.

Not to say that you couldn't have 2 connections, but it wouldn't really help you since data has to be sent and recieved through one of the connections, not both.

I was wondering just exactly how to bridge two internet connections. My friend wants to get a second internet connection so that his family can use the netflix they signed up for and he can play games and torrent files without interrupting their streaming.

What do I need to do to help them set this up? I was told I needed to bridge the connections. Is this true?

If your friend wants 2 internet lines/connections then there are considerations.

2x ADSL connections will require 2 phone lines to the premises otherwise the speed would still be limited to that of one line, and you can only connect one ADSL modem per line due to how it is connected at the exchange.

If it's two cable lines then no problems there, assuming ISP and routes/street cabinets have adequate bandwidth.

So If your friend wants to use his own connection whilst remaining isolated from his parents then the best thing to do is simply plug one of the connections into his gaming device(s) directly, or purchase a second router to use for these devices (if WiFi remember to use different channels).

"2x ADSL connections will require 2 phone lines"
That is not strictly true. You can have dry loop. In Canada, Bell offers dry loop for free if you want Internet but do not want a telephone line. Even when you buy Bell;e service via a retailer such as Acanac or Primus, you can get dry loop for an additional $8 or so over and above the cost of Intennet ADSL connection.

"If I bridge a 6mps cable and a 4mps dsl will I be connected at 10mps?"
I do not know about mixing cable and DSL, although more than likely it is possible. However, in Ontario and Quebec, Canada, a service call Acanac offers MLPP. This service allows you to subscribe to two or three lines (or dry loops) and two or three Intenet accounts merged into one high speed account jumping in increments from the basic 5-6 mbps to a miximum currently of 24 mbps.
[URL="http://acanac.com/mlppp-internet.You can read about it here-internet.html[/URL]
From their site, [I][COLOR="Green"]"A new type of High Speed Internet. MLPPP for the Masses!
MLPPP connects your home or busniess to the Internet through more than one link. MLPPP is right for you if you want increased speeds and reliability."[/COLOR][/I]
I think they have a service in the States but I do not know the details. I trust this partly answers your question, and hope you will investigate more and post your experience.

The continuity / reliability of your Internet connection increases when adding a 2nd connection, but not in speed.

Either connection can be used as a fallback when the other connection is dead. Assuming both connections are technically based on a different independent internet infrastructure.

There are routers which have a several layers of fallback switching down from ADSL to wireless 4G (Winax) to 3G(HSPA/EVDO) to GPRS(2G), such as this netgear router.

I used to run a server with Sygate software router allowing several layers of seamless fallback when the main connection fails. It was based on 2 physical separated internet connections such as a Cable Internet, an Adsl connection and a simple modem (non-adsl) dailup. The only different the user noticed was speed.