Why would 2 camcorders not be ideal for your needs?
Unless they are unattended you wont miss anything if you allow a 1-2 min overlap which can easily be sorted during editing
otherwise capture "live" to HDD on a Laptop . I would rather a laptop than the firestore devices as they work out much cheaper. £299 gets you a half decent new one these days
but
2 Camcorders ( one could be loaned ) works out much cheaper
However to answer you directly , there is no MiniDV tape changer ( domestically or indeed in the industry
AFAIK)
You will be surprised how many 2 hr programs you watched have been the result of very skillful editing. In fact you could record the audio ( by whatever means) use a few stills, and then use the next tape in same camcorder to get a seemingly seamless recording