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I would just get some mozart flute duets or something like that and play the second flute part.

Ken Amis wrote a duet for flute and tuba called Inflections. It is 7 or 8 movements in length, none of which are particularly long. It is not terribly demanding for either player but it does have some moments that can be tricky for the ensemble. It is mostly audience friendly with a few extended techniques such as lip bends and circular breathing employed. My personal opinion is that this piece would work better on a flute recital because it (most likely) would be the only 20th century piece on the recital. When I performed Inflections this was the case. Also I believe Harvey Phillips did some Telemann duets which might be a nice change of pace on a tuba recital.

On the ITEA Legacy Series CD of Harvey Phillips, there's a recording of Harvey Phillips playing three Handel flute sonata movements with a flutist. Handel's flute sonatas are written for soloist with basso continuo, so there's a solo line and a bass line written out. On the recording, the flutist plays the solo line and Harvey Phillips plays the bass line, which is very active and challenging (i.e. arpeggios up to high F, which he makes sound easy!). I've played these in recital, as well, and really enjoyed playing them.

Kent Eshelman

I highly recommend the Beethoven Duos for violin and cello. The flute plays the violin part and...well you get the picture. It requires some delicate playing but after a whle the great music starts to come through. Good stuff and quite challenging.

Roger

My girlfriend is a flutist and we have worked up a lot of duets. There are a number of Haydn pieces written for flute and continuo (some have been recorded by Harvey Phillips) these will usually require you to read the cello part, we have also done some Teleman and some other miscellaneous duets she had where I transposed my part.

If you are looking for something written specifically for flute and tuba try the Duet for Flute and Tuba by Walter Hartley. It is not terribly difficult but it is fun to play and will require a little practice on your own and with the flutist.

TMB

Bach 2 part inventions work great.