A Corporation Tax accounting period can't be longer than 12 months. For example, if your company accounts cover an 18-month period, and your company is trading throughout, you must submit 2 Company Tax Returns because you'll have two Corporation Tax accounting periods. The first accounting period covers the first 12 months and the second period covers the remaining 6 months.
You only need to include your company accounts - in iXBRL format - with one of the Company Tax Returns.
Using HMRC's Corporation Tax online filing software
If you're using HMRC's online software, the accounting periods will be listed based on information held by HMRC, but you may need to change them. To do this,see the 'Change Accounting Period' function. You can then select and download the online return form for each accounting period. But you only need to complete the accounts template for the first accounting period, along with the computations for the same period.
For the second and any subsequent accounting period:
Using commercial software
You must tell HMRC what your accounting period start and end dates are when you submit your return(s).
If you're using commercial software, you can attach one computation for the entire accounting period to one CT600 return form (with matching return period start and end dates). If you do this you'll need to include an explanation on the CT600 form that the computation is not included because it's included with the other form.
Alternatively you can attach the 'same' computations to both CT600s by changing the dates reported for the return period start and end dates in the computations or attach a separate computation to each return covering the accounting period.
Please contact your software provider for advice if you need more information about how to use your software.