I would think that you would have to have two DSL modems to do this. I am not personally familiar with new DSL setups however. Also, if you can do this, most likely your ISP would require you to purchase two public IP addresses for those two systems. The price for that varies. Usually what I see is $10 for another IP.
A router would be more secure than having your PCs directly connected to the Internet (unless the DSL modem also provides a hardware firewall/NAT and granting if you have your Wireless network encypted if you got a wireless router) and probably less expensive over time. Depending on how much down bandwidth you have, you probably won't notice a big difference if you are just surfing the web. I have personally been downloading at full speed on one of my PCs and not really noticed a big difference browsing the web on another PC.
May I ask why you want to do this other than getting a router?
Hope this helps...