Maybe a "why would you want to do that" issue?

The easiest way is to not lock the front hubs or axle. Why? Sometimes you want to go slow over rocks etc. and turn sharply when traction is good. No mention of the transfer case type or axle type complicates the answer but I hope it helps. The fact that you do have 4x2 in high range tells me you can unlock the front axle.

This is a design feature intended (so I've read) to require that both axles help to utilize the increased torque available in low range/low gear use.  Its pretty easy to twist off an axle with overzealous throttle in 2LO if traction is good.

It's a simple matter on many transfer cases to convert to "twin stick" operation which allows both 4x4 and 4x2 low range.. e.g. external linkage change only.

It fairly common in serious off road vehicles... I helped a friend build up an early '60s Bronco for serious off roading that has the twin stick conversion.  

Front hubs are typically locked at the trail head and left in that position for the day; you don't want to get caught with a wheel in deep mud, unlocked hub, and needing 4x4 to move forward. With twin sticks, selecting 4x2 or 4x4 in low range is as easy as a gear shift.  

4x2 low is frequently used for running trail sections where 4x4 not needed, but need low range for better speed control.. On a loose surface with lockers in the both axles, the vehicle tends to push forward/wide in a tight turn. Putting it in 4x2 (still needing low range) helps reduce this.   

as someone said. to help even out the loading, plus 99.99% of people would never use it.

there are always mechanical or electrical means of bypassing this safety feature though. locked hubs or some vacuum bypass for mechanical, and then there are electrical means that are fairly simply to implement which you will find on enthusiast forums.