Let's break down the problem step by step:

Given information:
- Three men or six boys can complete a piece of work in ten days, working seven hours a day.

First, let's find the work capacity of one man or one boy in one day:
- Three men can complete the work in 10 days, so in one day, they complete 1/10th of the work.
- Similarly, six boys can complete the work in 10 days, so in one day, they complete 1/10th of the work.
- Therefore, the work capacity of one man or one boy in one day is 1/10th of the total work.

Next, let's find the work capacity of six men or two boys in one day:
- Six men working together can complete the work in fewer days than three men, as more people are working. Let's say they can complete the work in 'x' days.
- Similarly, two boys working together can complete the work in fewer days than six boys. Let's say they can complete the work in 'y' days.
- Since the work capacity of six men is twice that of three men, they can complete the work in half the time. So, 'x' days would be 10/2 = 5 days.
- Similarly, the work capacity of two boys is twice that of six boys, so 'y' days would be 10/2 = 5 days.
- Therefore, six men or two boys can complete the work in 5 days, working seven hours a day.

Now, we need to find the number of days required to complete a piece of work twice as large with six men and two boys working together for eight hours a day:
- Since the work is now twice as large, it means it will take twice the amount of time to complete.
- So, the number of days required would be 5 * 2 = 10 days.
- However, the workers are now working for eight hours a day instead of seven hours.
- To adjust for the increased working hours, we divide the number of days by the ratio of working hours, i.e., 8/7.
- Therefore, the number of days required to complete the work would be 10 * (7/8) = 8.75 days.

In conclusion, it would take approximately 8.75 days to complete a piece of work twice as large with six men and two boys working together for eight hours a day.