Depends on what you mean by "stereo input", and how you hook everything up.
If the first Eventide is able to send dry and wet signals to separate outputs, then you don't need an external splitter. But if it can't, then you do. Either way, the 100% wet channel is going through the second Eventide, and then you have two mono channels, one wet and one dry.
If you have a mixer with a stereo input that's a single TRS jack, then you need to get a special Y cable to go from 2x TS (mono) to 1x TRS (stereo).
If you have an amp that has two channels that mix to one speaker box, then there's your mixer, you don't need another one.