Having two PC's connected to your wireless router which in turn is connected to your broadband modem WILL NOT reduce the internet speed that your modem usually achieves. I trust you are aware that if one of your PC's is downloading data at 200kbps (lets say the server you were downloading from allowed you to download at this speed..) then your other pc will only be able to pull down the remaining bandwidth of your internet connection.. it will not give you 2 x 2mbit connection, it IS ONLY 2mbit shared between two PC's.
If you were asking whether there were any side effects to connecting 2 pc's to your 2mbit connection, then the answer I think you will be pleased with is 'No'. If only one of your PC's is actually switched on, you will find it can use the entire bandwidth of your internet connection (though you may not actually get a true 2mbit download speed as servers which host web sites generally 'throttle' your download speed - which is why it is more economical to perform several downloads at once from different servers (presuming you have the need to get several things done at the same time..)
If you use your 2mbit connection for gaming I can reassure you that your gaming will be uninterrupted by having another computer connected to the router - I have 3 pc's at home and often invite friends around to play online games.. RavenShield works wonderfully being played on these PC's simultaneously through a 2mbit ADSL connection, I use ZoomAir wireless network cards that connect to a Belkin 54g Wireless Router which plugs straight into the back of an X3 Zoom ADSL broadband modem. The only issue I 'had' was I also own a Panasonic Wireless Telephone, I experienced intermittent wireless networking until I discovered I could eliminate it completely by switching off my telephone!
Hope this helps,
Regards
Nick