WOW . . . I mean . . . just WOW.
I ~suppose~ what you're trying to do ~might~ be possible.
Once connected, it ~might~ be possible to bridge the two connections. Of course after you set up "Internet Connection Sharing" - and the like.
I've never even imagined trying to make something like this work. But please be aware that you're going to break every terms of usage agreement you've setup with your ISP itself, and if they find out about it, they could drop your service completely.
But legalities aside, if it ~were~ even possible, the physical limitations and stress to that little dial-up line would absolutely kill any sort of data throughput you had in mind, and most likely EVERYTHING would time out, and the other user wouldn't be able to web-surf, or pretty much do anything else.