That is strange. I can't think of anything other than something being crossed on the plug side or that some strange signal from the speed sensor is leeching it's way to the cluster and thus making both needles move - like a grounding issue.
EDIT: On 2nd thought, it could be that your car being a 98 might be the year where they switched plugs over and thus a ver.5/6 cluster (with green plugs on the top vs ver.4 with white plugs on the back) is the cause. 99-01 would otherwise be plug n play. There's a write up on how to repin your cluster plugs to make it work properly.
I had iwire do my harness merge and wired up the cluster for me. Not sure why i'm getting this issue at this point. Stupid polar vortex is making it tough for troubleshooting as it is so cold to work out side to find out what is going on. You are the second person to think a grounding issue.
No one is 100% sure at this point of what the deal is. It is a rather strange one.
EDIT: gauges acting weird with the originals in place - https://www.youtube.com/watch?v=j7GuozI7YQo#t=32 - this was when the 2wire vss was hooked up and I would expect that it would have read 0 acrossed the board along with the tach being off.
I don't know why you didn't consult Iwire right off the bat if he did your harness merge. This is most likely a wiring issue and he would be the one to help you.
There are way to many variables for any of us to easily diagnose this problem. What was merged with what? What VSS are you running now? What color are the plugs originally on your cluster and what color are they now?
I quickly read through this and it seems you have a green plug cluster in a white plug car with a merged harness using a different VSS than factory for the cluster? Is this correct?