You get more uses.
According to the Minecraft Wiki article on it, the formula is:
floor( Item A uses + Item B uses + (Max uses / 10) )
So not only do you save inventory space by combining ("repairing") partially used tools, you actually get more uses out of the combined tool.
Yes, but with rapidly diminishing returns
According to the wiki page on Enchanting1, the effect of Unbreaking is:
Increases durability
[...]
For armor, (60 + (40/(Level+1)))% chance a use reduces durability. (In
other words, each durability hit against “unbreaking” armor has a
20%/27%/30% chance of being ignored.) Thus, on average, armor lasts
25%/36%/43% longer.
You can see that the formula is different for armor, and even with the regular three levels, the chance to not lose durability when hit increases slower with higher levels. Extrapolating the "20%/27%/30%" using the formula given yields:
Unbreaking | 1 | 2 | 3 | 4 | 5 | 6 | 7 | ... | 10 | ... | 20
Chance | 20 | 27 | 30 | 32 | 33 | 34 | 35 | ... | 36 | ... | 38
Increase | 20 | 7 | 3 | 2 | 1 | 1 | 1 | ...
(Values are rounded in the table, but not in game). The chance converges to 40% for high values of Unbreaking, with diminishing returns the higher the enchantment level gets, to the point where is negligible.
If you directly compare an Unbreaking III Diamond Chestplate to an Unbreaking IV one, on average, the former breaks after 529*100/(60+40/4)=756 hits, the latter after 529*100/(60+40/5)=778. That is only a relative increase of 3% more hits.
1 While the page has changed, the game mechanics have been the the same since at least March 2013.
Best Answer
Unbreaking causes tools to take durability damage only
100/(level+1)%of the time. For Unbreaking I, tools take damage 50% of the time, so your effective durability is 2x. For Unbreaking II, tools take damage 33% of the time, so the effective durability is 3x.2 Unbreaking I shovels would have an effective durability of 4x (since there's 2 of them at 2x each), wheras the Unbreaking II shovel you could combine them into is only 3x.
So, if you want to go for the hyperefficient analysis of the durability, yes, 2 Unbreaking I shovels will last a little longer overall than one Unbreaking II shovel.