It is permitted.
A problem of כסדרן may appear - Rm"a addresses this (OC 32:1) and says it should be taken care of לכתחילה, but if not it's not פסול.
Regarding Rashi & Rabenu Tam - it would be contradicting to wear Yad of one and Rosh of another. However, Rabbi Ya'akov Medan says it is לכתחילה to wear rashi for Yad and Rabenu Tam for Rosh.