It is currently not possible to use individual discount codes of a promotion together.
This means, for example, if you send a customer 4 individual voucher codes with a €50 discount each so that they can redeem their €200 total discount either split up or several codes together, this will not work if the individual discount codes all come from one promotion.

However, if the individual discount codes come from 4 different promotions, the customer can redeem them in combination.
This function would be a useful extension for many customers, for example by adding a button in a promotion asking whether the codes can be redeemed together. There have already been several issue tickets about this behavior in the past, as many customers do not understand why the redemption of different promotions together works, but not of a single one.