According to me to double the speed is not possible by the way u suggested..........
1.) in order to connect the 2 pcs u'll need a connection between the 2.... which will act as a bottleneck.....
2.) moreover u can install a software such that the data U send will be managed........ but what abt the data that the server sends u.......which way will it send?????to ur pc or ur friends pc.....
3.)then there will be security issues.......if a pack of data is sent thru ur computer to his ..... then u can view it....and change it....

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see its not as simple as 2+2=4....