Yes. Your fuel level senders and oil temperature sensors can not be connected to two instruments at once, since these parallel connections interfere with the ultimate voltage output of the sensors.
Incidentally, even if you could figure out a way to get everything reading correctly simultaneously, you'd have the reverse problem. Namely, the failure of EITHER instrument would cause the voltage output of the sensor to change, and then the remaining instrument would then read incorrectly. So what you think is redundancy (again, assuming you could get everything working when simultaneously connected, which you can't) isn't, but in fact adds an ADDITIONAL potential point of failure.
Unless you have an exotic manually controlled turbocharged engine that requires constant knob fiddling to keep it from exploding, one set of engine instruments is enough. If your engine instrument(s) fail, the engine keeps running, and the sound of engine tells you everything you need to know. Basically, engine instruments are rarely critical enough to warrant redundancy.
