If you use a Y-lead to go from your TRS to two TS jacks, this will give you each pickup into one of the inputs, the TF will not sum the inputs – it will operate in true stereo.
If you actually want to sum the pickups, you will have to find another way. First thing to try would be to parallel them. If this spoils the tone, next step is a passive mixer cable (two resistors – this will cost some level).
Note that each of the TF inputs are mono TS, not stereo TRS.
