That's a breakthrough, because modems have already reached the physical limits of analog phone lines. Buying a faster single-line modem no longer speeds up your throughput because the line itself is the bottleneck. The latter would likely be about double the cost of a single-line connection. On top of that, the local phone company charges you ISDN connect rates that average 2 cents to 5 cents per minute. Modems that support multiple analog lines sound wonderful, but there's a hitch.
Many Internet providers have hundreds of ports that answer on a single phone number.
If your multiline modem connects to two or more of those ports at once, the provider's system must know which two are yours, so it can divide and route the data properly. Most providers aren't set up for that. This problem could be solved with software so providers would not face expensive hardware upgrades. But it's a much different approach to the way they do business. I suspect they'll change only if there's a strong demand for it, and that's not going to happen overnight.