I assume that the FIRST 20 channels are for incoming
and the LAST are for your outgoing
In your TDCSU for the Incoming channels change the circuit as follows:
CHANGE-TDCSU

EN=X-X-X-X,BCGR=1,SRCHMODE=ASC;
For the Outbound last 10
CHANGE-TDCSU

EN=X-X-X-X,BCGR=2,SRCHMODE=DSC;
In that way incoming calls will always try channel 1 first and then 2 and so on.
Out bound calls will always try channel 30 first and then 29 and so on.
If they 'meet in the middle' you have a decision to make,
Do you have enough channels for the site?
Would another circuit be advisable?
You could stop incoming calls alltogether on the 2nd BCGR but have you considered what your customers would think if they get a busy signal?