I would like to inquire which is logical to take when taking the CCNP track? Is it the 642-902 or 642-813? I am assuming the troubleshooting must be taken last as it would require both knowledge to pass
If you are more familiar with switching, study hard, do labs, then take the 642-813 test first by all means. If you are more skilled in routing, then go for the routing first - study, do labs, test.
The TSHOOT will test you on both switching issues and routing issues (as well as trouble shooting techniques!). The TSHOOT uses a "trouble ticket" format so you will need to get the book, study the scenarios, and be prepared to solve the cases that the test will present to you.
Best of luck!
If you want a logical answer, take 813 before 902. As of OSI layers, you learn something at L2 then at L3.
But, of course you can go with the instinct.