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The easiest thing to do is get a router. If your laptop support wireless internet, get a wireless router but make sure to set encryption on it so you don't become the neighborhood "hotspot"...
Your modem talks to a single modem on the other end of your phone line at the phone switch office. Almost surely two modems talking at the same time would confuse the other end, but I am not up to date on DSL technology and protocol.
In anycase, there is almost surely no need for two modems driving one line since a wireless router will handle just about any need. Unless you are a heavy duty gamer and need maximum speed on each line, the wireless is fast enough, even for videos.
I have been able to use a wireless modem between our two warehouses, a block apart. The signal is weak but it works. You can also buy range booster antennas for the wireless modem, this can often boost the signal enough to overcome problems. Range is very dependant on the router, and the receiving PC and interferance between the two.
I am pretty sure you can get at least a couple hundred feet since I often am able to connect to neighborhood modems if I am away from my home or office.
It takes a little digging, but for point-to-point wireless applications, one can buy directional antennas that can incease WiFi range to nearly half a mile. And the newer cards and routers use some sophisticated signal processing techniques to increase that range even further.
Use a wireless router and if your laptop or desktop does not have wi-fi you can get a card to plug in to a usb port. That would be the easiest and probably the cheapest. If using a laptop it would also allow for use at other locations ie: under shady spot, porch, etc.
A second modem would not connect anyway. The modem has to be provisioned by the mac address in the ISP database. If you just stick another modem on the line, it will try to connect but a connection will not happen.
As stated above, you can go wireless router or wired and pull cat 5 cable to the garage.