am I losing a lot of signal splitting the lefts and rights coming out of the preamp
You are not "losing signal," but there is a possibility that you could be introducing some degree of frequency response irregularity. There are two possible issues:
1)The GFA-555 is spec'd as having an input impedance of 22K, which is lower than ideal. Assuming the GFA-555II is also 22K, the preamp output for each channel would see a load of 11K. Preamp output impedance should be less than one-tenth of that amount at all frequencies (some would argue for a smaller fraction than that), or significant frequency response irregularity may result.
I could not find specs on the output impedance of your CT-17 tuner/preamp. Since the CT-17 is solid state, I would guess that it has an output impedance of well under 1.1K at all frequencies, but I can't say for sure.
2)The total of the capacitances of the interconnect cables to BOTH amps will affect the signals going to BOTH amps, by interacting with the preamp output impedance, resulting in some degree (probably small) of upper treble rolloff, IF the cables are long AND they have high capacitance per unit length AND if the CT-17 output impedance is high.
If your cable length is not particularly long (say ten feet or less), I wouldn't worry about this. If they are much longer than that, I would make a point of using low capacitance cables. How low is low enough depends on the length and on the preamp output impedance, but as a rough general guideline I would say aim for 30 picofarads (pf) per foot or less if your cables are long.