Omni lav plus what for redundancy?
 

I'm going to shoot some inside interviews, probably 1 person sitting or standing in one place. Possibly 2. I'll probably use my Sennheiser ME102 wired lav for one person (because I have one) but want to use another mic on the other channel for redundancy. I'm assuming a cartoid on a boom. I was considering buying the ME64 because I have the ME102, ME62 and ME66, so I only have to pick up the cartridge.

Maybe something else would work better for about the same $ ?

Do you have phantom power or are you running the two Sennheisers on battery power?

Phantom
 

Phantom power from the camera, although both have battery power available if I want/need it.

It's going to depend on the locations you record in, how vocal your interviewees are, how you plan to frame them in the shot and how good your camera's mic preamps are.
The ME64 is a very sensitive mic with a wide open pattern and a peak in the high frequencies. If the person is quiet, and your preamps need a mic with a lot of gain to be at their best and you have low ambient noises, then the ME64 can be a very good choice. If the conditions aren't like that, then those qualities can work against you.
It will be the same for any mic at this price point that each will have strengths and weaknesses.
For about $15 more than an ME64 capsule, you can get the AT3031 (a complete mic). I think the AT3031 is a much better sounding mic than the ME64/K6 generally, but if you need really high gain and battery power, then the ME64 would be better.
The Rode NT3 is now more expensive than when I bought mine. It would be a good choice but I don't know what you want to spend. It has a good pattern and frequency response for indoor recording, as well as battery if you ever need it, but it has much lower sensitivity. The mic has low self-noise but if your camera preamps are noisy, then it could be a bad choice.
So think about where you'll be shooting and how good your camera preamps are and make the best guess.
If your space has soft acoustics and isn't reverberent, you can use the ME66. If you're in a small, harder space and the person talks loudly, then even the lav isn't going to sound very good although it will be much better than the ME66 or any mic placed outside the video frame.