Splitting a +5V rail into 2 isolated +5V lines
Isolated (galvanically) means "no direct conduction path between the two sides" and imply "floating", which is in direct contradiction with "share a common ground". However, isolated does not imply that if there is a high current load, the voltage does not drop, which is your problem here.

So "isolated" is not what you are after.

What you are after is a more stable supply. Even if your supply is correctly rated for the total current needed, it may not be able to cope with high current transients without the voltage dropping. The servos need higher current when they start moving.

You can add a capacitor close to the Pi to overcome this. A big electrolytic could be appropriate. Its minimum value depends on the layout, peak current value and duration, but something like 1000uF will most probably fix it.