Yes you should be able to have more than one sensor on a but at the same time. In fact in theory you can have 128 of them.
Are the addresses different?
Have you got two sets of pull up resistors or even no pull up resistors?
Thank you Grumpy_Mike for the hint with the pull up resistors.
On the 9DoF run the Bus with 4,7K to 3.3 Volt and on the RTC with 2,2K to 5 Volt. Thats can not run.
So I have an problem what can I do? Can the DS1307 RTC run with 3,3 Volt on the I²C or have I an other chance to connect to I²C Buses with different Voltages?