Depending upon where your grandparents live, the cost for 12 hours per day of a home health aide and the cost of an assisted living facility for both grandparents may
be about the same. For example, near or in a big city a home health aide might cost $20 per hour and you could expect the cost of an assisted living to start at $3500 per month per person. Using these estimates you'd pay about $240 per day for the home health aide (12 hours x $20) which would amount to $7,200 per month if the care was provided for 30 days. This is roughly the same as the cost of moving both of your grandparents to an assisted living facility.
If you haven't already, I'd strongly suggest calling a few home care agencies and assisted living facilities in your grandparents' community and then doing some similar calculations. Be sure to take into account days of the month where family may be willing and able to step in so that 12 hour assistance isn't need and also consider how your grandparents' needs may change over time and require more that 12 hours of care. One way to assess this is to connect with their physicians and ask that very question - "Without using your crystal ball ;), how might you expect my grandmother's or grandfather's health to change in the next several years and how would those changes impact the amount of care he/she might need?"
Lastly, I want to add here that cost really should be only one factor as you begin to plan your grandparents' care with them; where they would prefer to live should be first and foremost in any discussion and every effort should be made to honor those wishes for as long as emotionally, physically and financially possible.
All the best to you.