can i combine two computers and use them as if they were one computer?

This is only really feasible or useful if you have a large amount of basic calculation or 'data crunching' to do. It requires software which will manage and assign the tasks to each computer. In fact, each computer will be processing its tasks simultaneously but entirely separately. Once each machine has finished the task allotted, the results of the calculations will then be combined and another task assigned. In this way, very large calculations can be done in a far shorter time. This is, in fact, the principle behind such programs as CashGopher, GomezPeer and the now defunct SetiAtHome. Most normal applications such as Word Processing, Spreadsheet and Graphics programs could not be used in this way or, at least, would not benefit from it. Very large 3D rendering/ray tracing programs, such as those used for animated films, can work in this way and, in fact, are often designed to do so. The sort of work involved is way outside what most normal home users would ever need or want to do.