Monitor for allocation contention. That's all that multiple files and traceflag 1118 are for, if you don't have allocation contention then there's no point in adding the traceflag.
Ed's right, with Express limited to a single core it's incredibly unlikely that you can generate the concurrent load required to cause allocation contention in TempDB.
Are you seeing allocation contention? If not, why are you considering traceflag 1118?
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