Both sentences are grammatically correct. However they are both semantically problematic.
The problem is with the future tense of the second clause. Let's look at it in isolation:
I will not be sure when I will come back.
This may be true, but unimportant. Whether you will be, at some point in the future, sure when you will come back is irrelevant to what you are trying to communicate. What you are trying to communicate is the fact that now you are unsure when you will be back. ("Now" meaning when you are writing the message. It doesn't matter, from the point of view of the message, whether you will become sure at some point in the future.) To communicate this you use the present tense:
I am not sure when I will come back.
Putting this back into the compound clause:
I will be out of the office and I am not sure when I will come back.
The second clause implicitly takes the same subject as the first if it doesn't have one so you can omit the "I". However you cannot omit the verb, as it will implicitly be taken as the same as the first clause ("I will be") which is wrong.