Yes they had one but they dont sell it anymore. The only options u have and if u r going to share ur internet access is to buy a webramp, an Internet Access Server from multitech or u can simple put 2 internal or external modems in ur PC and use the multilink option that windows have. The last option is the one if u r not going to share ur internet access.
Diamond did make one, I think I still have one around here somwhere. It's two modems on one ISA cardand it will multilink under Win98 if your ISP supports it. It didn't work well for me, one of my phone lines is just so bad it was always holding things back.