Celeron CPUs were usually CPUs that shared the same core architecture as the current Pentium standard, but often had a lower core clock speed, lower core memory speed, and/or had smaller L2 caches.
Workloads have different constraints however, and simply doubling cache, clock speed, or memory bandwidth doesn't necessarily double performance, especially when running more than one application at once. Keep in mind, this is Windows 98 /NT/2000 era here.
Symmetric multi-processing (SMP) could be of huge benefit however, far more than simple doubling any of the above factors. Running two threads at once was unheard of on the desktop. These were usually reserved for higher-binned parts, like full-fledged Pentium workstations and Xeons (usually the latter.) But Abit's board gave users a taste of that capability on a comparative budget. Were two cheaper than a single fast CPU? Probably not in all cases (depends on speeds). But Abit's board gave users an option in between a single fast Pentium and a orders of magnitude more professional workstation: A pair of cheaper CPUs for desktop SMP. And that was in reach of more people.
In short, two Celerons were probably more expensive than a single fast Pentium, but having SMP meant being able to run certain workloads faster or more workloads at once at a time when any other SMP system would have cost tons.