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.