Your Flowering Crabapple should have formed orange-yellow fruit after it flowered....so something happened to prevent this. Did you have a cold spell or high winds that might have caused the blooms to fall prematurely? Unfortunately....you will not have fruit this year.....sigh.....but hopefully you will the next.

You certainly can remove some of the apples off the branches of your other tree. This actually is a good thing to do. It allows the remaining fruit to have the space to develop....and there will be less chance of damage to the branches if they are not weighed down.

A five in one apple tree is an example of multiple-variety trees that have 5 varieties of apples grafted into a single trunk and rootsock. A 3 in 1 or 4 in one or 5 in 1 tree gets you a selection of apple tree (and pollination as well).

All apples require pollination although most will set adequate fruits on their own, but will set more fruits if pollinated by other variety.

If you have only one tree and your neighbor has a pollinating variety, you don't need to plant another one. If your tree is not bearing, you can graft a branch from another variety into it, or place a fresh bouquets from another variety (in a can of water) at the base of your tree. Don't use bouquets from a triploid.

Certain varieties (triploids) are pollen sterile (ex: Gravenstein or Winesap)and do not produce fertile pollens and will not fertilize their own flowers or those of other apples.

What variety is your green apple?