Assuming you are using WinXP, yes very easily. You need a network card (about £10) in each PC and connect them together with an ethernet cable with one PC connected to your ADSL modem or alternatively use an ADSL router which allows you to connect up to 4 PCs directly to the ADSL modem.
Go to this site for info on options available and how to set up a simple network and to share your internet connection: http://www.homenethelp.com/
You can set up a simple network with Win98 but it is not as easy as with WinXP which has networking setup "wizards" already built in to help you with setting it all up. In Win98 you do have to understand a little more about the Windows software components you are using for the network and if you are not particularly computer literate, this can be a little complicated. Have a look around the site I gave you as that explains your options for each operating system.
You may find it easier to acquire Win XP for one pc and keep the other on Win98. That is the setup I currently use sharing the internet connection on a 2 PC network.
Your other option is to use 2 monitors on 1 pc. This requires a special graphics card which supports dual monitors such as a Matrox card.