I translate similar content for one client into both Spanish and Catalan, and I always use AnyTM to ensure consistency between the two languages. However, that causes the following problem:
Imagine, I have to translate T-shirt into Catalan. This is both in my Catalan TM (Samarreta) and in my Spanish TM (Camiseta). Since I am using both TMs (with AnyTM), it will RANDOMLY PICK THE CATALAN OR SPANISH TRANSLATION. So, I have to go back and pick the other language all the time, which delays my translation process. The absence of the penalty for different languages also results in 100% matches for a translation from another language. This can inadvertently result in segments being translated into the wrong language, but showing up as 100% match.
The solution is simple and the infrastructure already exists: add a penalty for 'Different language pair (AnyTM)'.