You can only have one CSS on your SIP trunk. I suggest you use xlation patterns in CUCM to xlate the new number to your desired pattern. Just make sure the CSS on the sip trunk can access both the 096XX and 096YYY patterns
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"'Nature is too thin a screen, the glory of the omnipresent God bursts through it everywhere"-Ralph Waldo Emerson
What are you DNs defined as? Are they exactly as what you receive from PSTN? How many significant digits is the SIP trunk configured as to collect? If it is "All" then your Xlation pattern would simply match 096XXXXX and 096YYYYY, if you are collecting lets say only 4 digits and you have an overlap in the last 4 digits you need to change the SIP trunk sig digits to "All" so that you can use these xlations, if they do not overlap then there is no problem.