mult index on for each

Does anybody know a way to force the use of a particular index based on an if, then , else clause?

Exmpl: for each customer no-lock

where if v-logical

then customer.cust-no = v-cust-no

else customer.mail-cust = v-cust-no:

/**I want it to use an index called customer if

v-logical is true else I want it to use a index called mail-cust*/

Anyway to do that without having two for each loops?

Thanks Tony.