I figure the main barrier for multi-elemental simultaneous use(not FUSED use, you need bloodline bullshit for that), are:
-Elemental conversion is not simple unless you have a natural affinity. If you do, then its actually fairly easy, seeing as the Uchihas natural fire affinity makes a pretty hefty Fire jutsu a regular coming of age ceremony.
-Chakra multi-tasking is HARD, you can't use your normal mnemonics, because you have to learn to do all the components sealless, THEN you have to do both at once. Like juggling, juggling blindolded, riding a unicycle, riding a unicycle blindfolded, and then to actually do it in combat you're juggling while riding a unicycle while blindfolded
Rin has the first part eased, because of affinity for every element. The second part is still hell.
Though on that note the Rasengan is difficult in the same way, you need to use chakra shaping for three different tasks(Containment, Vortex, Turbulence) at the same time. And the Elemental Rasengan then has to form elemental chakra while doing the above.
Short version: Grind chakra control. Then chakra shaping. Then all the elemental ninjutsu.
