Does using two Hall effect current sensors in parallel affect accuracy?
Relative accuracy will stay the same.
You need to combine the two current outputs with the same gain.
If the current split between the two devices is not equal, and varies (either with time, temperature or from batch to batch) and the output gains are not equal, then you will have a gain variation.
The poor man's version of this is to use an unmeasured current shunt bypassing the current shunt. An even poorer version would be to use a metal conductor of some description bypassing the measuring shunt. However, if this metal is not matched in tempco to the current shunt, you will get significant changes of gain with temperature.