Indeed, PPP has a multilink feature which an bond packets from multiple modems into a single bitstream. Both the client (your machine - Windows 98 could do it) and your ISP needed to support it.

I remember trying this a few times while at my parents' store. My dad would tell me which phone line I could use that day - either the dedicated line used to process credit cards (if they were closed), or a spare line off the PBX. But occasionally, I was allowed the use of both - and luckily had a pair of PCMCIA modems with the dongles in different places - and could get speeds well above 56k.