Sure, as long as you're eligible for both. A Thrift Savings Plan, or TSP, is a special retirement account available to federal government employees. An Individual Retirement Account, or IRA, is available to anyone with the cash to open one (there are IRAs, Roth IRAs, and a few other varieties, but that's for another kyooenay). TSP.gov says if you want to have both a TSP and IRA, Uncle Sam is more than happy to allow it.
Need more information? Newspaper archives are one of the best online research tools.