That would be ill-advised, you would be breaking the license conditions for both bands by doing this. Not only is it not that simple to do, it's really not that simple to do properly. You could so easily end up transmitting a harmonic of your wanted frequency, causing interference or denial of service to a legitimate user of the band. Where they would be well within their rights to knock on your door and kick your teeth in, or just send the authorities round. Not to mention that you could end up interfering with a vital service without even realising it. Far easier to just buy a couple of transceivers that have been properly designed and licensed for use at 433Mhz don't you think?
FRS radios are almost impossible to tune down that far unless you are an expert hacker. The synthesizer VCO won't tune down that far and would have to replaced, the bandpass filters would all have to be retuned, and you would have to hack the microprocessor to change the synthesizer programming. Forget it.