Well, you didn't say what OS you're running. I'll assume it's a version of Windows, as you say that you tried Midpoint.
If you've got 98 or 2000 (or XP) then you have Multilink built in. Under Dial-Up Networking, right-click on your connection, go to Properties, and the last tab on the right is Multilink. Select 'Use Other Devices.'
Now, I'm pretty certain Win95 also has this capability, but it might have been OSR2, or the DUN upgrade. I doubt NT 3.5 has is (although it might), and NT 4.0 probably has it, but that might have been included in a Service Pack.
Here's a good link for question like this.