Is it better to add a transcript of a non-completed degree for a master's degree admission? Im getting my degree this year (in june presumably) in physics. And while I was getting it I was at the same time getting a degree in mathematics. (By degree I mean completing undergraduate studies in both of those things, I'm from Spain and I'm not sure about the equivalence...).

Now, I dedicated to physics full-time and by the time I will have finished physics (june as I said) I will have approximately 50% of my degree in maths. Independently of the maths degree, next year I will apply to some post-graduate studies outside of my country and I would like to know if I should apply with my physics degree or if it would make any significant (and positive) change in my application to say that I've also been studying half a degree on maths and include a transcript of what I've studied in that degree.

The problems is what I commented on BPND answer, plus the fact that transcripts are 30€ plus 80€ the sworn translation, and if I can keep expenses down, that's a plus. My question is if the difference by including it will be positive. The average grades are 9.24 (GPA 3.9) and 9.2 (GPA 3.82) in physics and maths respectively, out of 10.

The GPAs were calculated with some online converter I found, using all the subjects and the credits for each one.
Many universities will demand a full transcript of your academic history, so, in that case, you will need to attach both (or a combined) transcript(s) to your application.

Even if they only ask for the transcript of your main studies, you might want to attach the transcript for the math lectures and exercises. Especially in this case, where math acts perfectly as a supporting discipline for a phyiscs major.