Splitting 220v breaker to two separate GFI circuits
You tried to poach the second shed's neutral from the GFCI protection on the first shed, so its no wonder the first GFCI tripped
What happened here isn't as weird-sounding as you are thinking it is.  You just need to remember two fundamentals here: first, that electric current flows in loops (it always has to make it back to the utility, generator, battery, or so on) and second, that a GFCI has nothing to do with ground as it simply compares what went through it on the hot side with what went through it on the neutral side and trips if they don't match up.
This all adds up to this being a simple wiring error; namely, that you tried to tap neutral to the second shed from the load side of the GFCI in the first shed.  As a result of this, when you tried to use the receptacle in the second shed, you injected the charger's running current, taken from the red wire (which the first shed's GFCI has no clue about) into the load neutral terminal of the first shed's GFCI.  Said GFCI sees this inexplicable-to-it flow of current on neutral, goes &quot;WTF?!&quot;, and trips, just as it's designed to do, thinking something is wrong, which is true.
Fortunately, the fix here is simple; you just need to move the neutral connection going off to the second shed to the LINE side of the first shed's GFCI.  Note that you'll probably want disconnect-rated GFCIs here if you don't have them already (they're denoted by having ON/OFF labeling on their buttons to go with the normal TEST/RESET labeling).