T11 and T12 are file formats used to store tax return data from a software called At Home. You can view content of these files in the program itself, but a conversion does not make much sense to be honest. These files contains data from two separate years so not something you would usually convert, but maybe it is somehow doable in the program. Except for that, we have no information about any software capable of t11 to t12 conversion.
Open T11 file Open T12 fileA popular tax preparation software from HRB Digital
A complete a simple federal tax return software