Yes, you can do this, and yes, you can use most of the bandwidth. For lots of info, do a google search for 'channel bonding'.
The slightly shorter answer is there are 2 ways to channel bond.
1: If your ISP supports it, use Microsoft's Multilink PPP. Set up a second dialup adapter (I believe it's labeled something like VPN adapter). Then, when you set up a connection, the last tab is MultiLink PPP. click that, select your other modem, and give it a number to dial (If your ISP has a hunt group, just give it the same number). The best way to see if your ISP supports Multilink PPP is to set it up, and try to connect. Or you can ask, or browse your ISP's home page to see if they mention it. Some ISPs may charge extra for supporting Multilink PPP, but the doubled bandwidth is a nice feature.
I used to use this. I had 2 connection icons. 1 for single line, and 1 for dual line. I would use the single line during the day, but once the kids & wife went to bed, then I would bring up the dual channel. This method has 1 advantage. It does channel multiplexing at the tcp/ip layer, so both channels are used simultaneously. So, if you connect 2 channels @ 46K, then you get a 92K connection. Very Nice.
Which brings us to channel bond type 2:
2: For this you need 2 ISPs. Dial one ISP with modem 1. Dial the second ISP with modem 2. Then you need to run a little "switcher" task that constantly mucks with your routing tables. What this will do is constantly change the routing table entries to first point to ISP 1, then ISP 2 then back again....
This will effectively route browsing requests out both modems. It's not perfect, and the major drawback is that if you start a large download, you tie up one modem, and the download uses only that one modem. However, for normal browsing, it does speed up the process.
HTH