Charging a unit with no space?
If the two units are charging at the same time, then the two units must attempt to charge in such a way that an equal number (or as close as possible) of models from each unit engage the enemy unit (8th Ed Rulebook, pg 23).

If the first unit is already engaged (an ongoing combat from a pervious turn) to the front and completely blocks it, and your charging unit is in the front arc, then the rules are not 100% clear.

The rules for Flank and Rear Charges (8th ED Rulebook pg 21) state that "a charging units position when the charge is declared determines whether it charges into the front, flank, or rear of an enemy unit". Based on this, you must charge the side you are on, not the side you can reach.

The rule for Multiple Charges On A Unit say that if "more units can complete the charge than can fit in base contact with the target" then the charging player choosing which units get moved first, and any "units that cannot complete the charge against the target are counted as having made a failed charge". (8th ED Rulebook pg 23). 

Since they must charge the front, which they cannot reach due to other units, I would say they will fail the charge.