Jump can only be used to decrease fall damage if the use of Jump beget the fall. Thus both can be used so long as the character knowingly jumped down the slope, into the hole, etc. If they tripped and fell off a cliff, jump cannot be used to decrease fall damage. If they decided to jump off a one story building, they can use Jump to decrease fall damage, as they are supposedly more in control of the fall.
Tumble can be used during any fall (well, so long as the character is conscious enough to think to use the skill). Thus if the fall is uncontrolled only Tumble can be used, but if the character initiated the jump on their own (through the use of the Jump skill) then both are effectively used (Jump when the jump was initially made, Tumble while falling towards the ground below).
As for the question of using two skills in one turn - both are considered part of the character's movement. So long as there remains movement after completing one the other can still be used (so long as its use would not require movement beyond what the character has available for the round). I note that falling does not have a movement distance associated with it, but using either skill for a fall presumes the fall will use up the character's turn that round. Thus one might argue that they should not stack - that jump would use up all the time/distance the character has available for the turn, but I would not consider that point valid. The jump is the initiation of the movement - the rest is mostly freefall that might yet be modified by the Tumble skill. I would not, however, allow any actions other than the Jump and Tumble skill utilizations (other than the single five foot step either before the jump or after the fall - if the character is not prone due to damage taken).
With the minor synergy bonus to two grant each other (+2 / at least 5 ranks), it is not impossible (presuming max ranks, decent Str and Dex, perhaps a couple magic items) to be regularly making DC 30 Jump and Tumble checks by level 10. I guess this means getting down from four story buildings are no longer as much of an issue. . . .