I was thinking this was great, and was even going to add that you can put a second spell in the staff for 75%, and a third and more for 50%, so having a staff of CLW & Lesser Vigor & other 1st level healing spells would be really cheap.
I have never seen any possible justification to ever have a staff with more than one spell in it.
All the spells share the same number of charges.
For example, lets say you want a staff of fireball + lightning at caster level 10 (where both of these spells do 10d6 damage)
The fireball staff costs 3*10*750 = 22500 gp for the spellwork + 300gp for the staff itself. so 22800 total. and can cast 50 fireballs
The fireball + lightening staff costs 3*10*750 + 0.75(3*10*750) = 39,375gp for the spellwork and 300gp for the staff itself. 39675 total for 50 uses of EITHER fireball OR lightening. So, 25 of each, or 10 + 40, or 1+49, your choice.
You are much much much better off getting one staff of fireball, and one staff of lightning. If you can afford both at full price, you have 100 spells vs 50 at only slightly more money.
If you can't afford both at full price then you can make one of them at half cost and only 25 uses. so you get 50 lightenings and 25 fireballs for less money than 25 fireballs and 25 lightnings on the combo staff.
Or heck, for almost the same as the cost of the one 50 uses staff (22800 vs 23100) you can get two staves, one with 25 uses of lightening and one with 25 uses of fireball