I noticed my '94 F150 has 2 cat converters on a single pipe coming off a Y form the manifold. What would be the harm in replacing both cats with a high flow aftermarket cat? Any suggestions? I just installed a dual out Flowmaster 40 original series and want a little more sound!

 

Depending where you are, the harm MIGHT be that it's illegal. Deleting the cats is certainly illegal everywhere. But some areas (like CA) require replacement with look-alikes, in addition to work-alikes. Other places allow replacement with parts that work the same OR BETTER, even if they look different (like a single 3-way cat). Check all the laws that apply before you proceed.

BTW
Most discussion forums (including this one) prohibit discussion of illegal activities in the user agreement.