How long is a piece of string?
IATA/carriers in an alliance will often have an agreement between them whereby they will accept a coupon at face value, which would usually mean the prorate value of the coupon based on the through fare in proportion to the sector fares.
Where there is no such agreement coupons are endorsed, usually for a specific value. There are many ways of reaching that, or sometimes none, meaning that the passenger is inconvenienced.
Between LCCs I believe it is done on a one for one basis whereby they will simply give each other credits for 'x' passengers.