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Does anyone know of any practice mutes that can be adjusted for use on both C and F tubas?

Yamaha Silent Brass. Of course that depends on the CC tuba; it won't fit some 6/4 sizes.

Morning,
Someone may be able to help, more specifically, but here is my bit if it helps.
Here goes...

[a] Like all the peoples of the world there are such a wide variety of tubas and sizes thereof.
Best to be specific about what horns you have so as to get the best advice here.

Can you borrow a mute to try?
Understand this may be problematic in these strange times.

[c] Contact a reputable tuba dealer and ask your question.

[d] If all else fails then attempt basic photogrammetry...
Take an HD image of the side view of each horn.
This view should be square-on showing the bell taper, ie. where the mute would sit.
Get someone to hold a rule or something of known size in picture to give true scale.
Make sure whatever you use is held against the bell, avoiding distortion of true scale.
You can now use this image to test whether a mute would fit or not fit.
Of course you'll need basic sizes of each mute you are considering.
To start you off I have provided a couple of images of my Denis Wick 5519.
http://www.musica-orichalcinus.uk/5519%20Top%20View.jpg
http://www.musica-orichalcinus.uk/5519% ... 20View.jpg
I cannot guarantee that this will answer your question nor that it'll fit.

In the case of the 5519 the foam outer makes contact with the tuba, inside the bell, and holds the mute in-place.
The rest of the mute must clear (gaps all-round) and not contact the tuba at any point.

But double check before you buy as they are a tad expensive.
{ I will delete these images on 1st April next }

I took an original Yamaha Silent Brass mute and used duct tape to attach a folded towel around the bottom edge. I have used this setup to mute many sizes of tubas, from a Amati F tuba with a 14 inch bell to a Conn 25J with a 24 inch bell.

As mentioned above a Yamaha Silent Brass should work in both horns. If you go this way, make sure you are using a stand - it is heavy. I was using one for awhile but eventually opted to buy two Schlipf mutes.