Wynnd is on the right track.
I would adjust the hpf to a higher point on the violin, sweep the filter up until the violin no longer has any "body" to the sound, the back it off until it sounds natural again.
Leave more bottom end in the cello, especially if solo or just with the violin - if they're playing along with a full band with bass and drums you may need to hi-pass it higher to take care of the mud in the bottom that comes from multiple bass-type instruments.
A small (pencil-type) condenser should sound good on the violin, SM-57 or equivalent should be fine on the cello, or if you have a larger diaphragm dynamic mic on hand like the SM7 or one of Heil's mics I'd go with that (I love the PR35 on cello).
In both cases aim the mic at the f-holes, being careful to place is so both the playing range and resting position of the performer can be achieved without accidentally tapping the mic - classical musicians are less microphone-aware than our rock, folk & gospel players.