I suggest you have a `testfor` command that tests if both a dropper and a crafting table are on the ground.

What I suggest you do is run an `execute` command at either of the two items, and test of the other item is nearby:

    /execute @e[<firstItem>] ~ ~ ~ testfor @e[<secondItem>,r=<distance>]

As you can see, I added `r=` to test only for entities in a certain radius (if you need the items to be 3 blocks or closer to each other, type `r=3`, and so on.). You can then put the command into a command block and lead a comparator out of it that would execute the command for custom crafting.

Because you already had half the thing working, I figured out you already found out a method of testing for specific items. An example can be found [here][1], just in case.


  [1]: https://gaming.stackexchange.com/questions/304055/how-to-execute-a-specific-item-in-minecraft