If you don't want to go with a helmet mount then they will both have to be frame mounted.
To get a clear image you will want to use aftermarket mounts, my suggestion is a RAM system. Could cost between 30-50 depending on what you go for.
For the best quality for money then the Contour ROAM is a great buy.
coming in at the moment at £139 on amazon (these were as low as £120 last week). Full HD, waterproof, and comes with a 1/4inch camera mount (the threads aren't great, so don't over tighten!)
These can be mounted like this
and like this -
So just over 300 for a good setup. If you need more information about which mounts to buy and were from, let me know and I can provide product codes. Note you also need to think about memory cars, 16gb on amazon for less than a tenner x 2.