Short answer: Yes, they can have both. TRICARE pays after most other health insurance, so the employer plan pays first and TRS pays second.
TRICARE Reserve Select (TRS) is a premium-based plan for qualified Selected Reserve members and their families who are not on active duty orders for more than 30 days, not covered under the Transitional Assistance Management Program, and not eligible for or enrolled in the Federal Employees Health Benefits program. Individual Ready Reserve members do not qualify. Members pay a monthly premium, an annual deductible and cost-shares.
TRICARE pays last after most other health insurance, including employer-sponsored plans, so the employer plan processes the claim first. If the employee disenrolls from TRS, they cannot reenroll for 12 months, so dropping TRS to save money deserves a look at the whole picture first.
Two cautions for the employer side: a reservist covered by TRS is not HSA-eligible, and the federal ban on incentives to skip the company plan (employers with 20 or more employees) protects TRICARE-eligible retirees and their families, not a drilling reservist covered only by TRS.