Tensor product notation convention? For two particle state, the Dirac ket is writren as
$$\lvert\textbf{r}_1\rangle \otimes \lvert\textbf{r}_2 \rangle. $$
Then how do we write its bra vector,
$$\langle\textbf{r}_1\rvert \otimes \langle\textbf{r}_2\rvert ~~\text{or}~~\langle\textbf{r}_2\rvert \otimes \langle\textbf{r}_1\rvert ~~\text{?} $$
Is there any rule or convention? I'm just asking the order of bra vector.
 A: The way I imagine it is that the left side of the direct product is exclusively reserved for hilbert space 1 and the right side is for Hilbert space 2.
So that the total hilbert space you are working in is written as:
$$
H=H_1⊗H_2
$$
And so when you have a wavefunction in H you write:
$$|\psi\rangle = |r_1\rangle \otimes |r_2 \rangle $$
And then:
$$|\psi\rangle^\dagger = (|r_1\rangle \otimes |r_2 \rangle)^\dagger = |r_1\rangle^\dagger \otimes |r_2 \rangle^\dagger $$
$$\langle\psi| = \langle r_1| \otimes \langle r_2| $$
Hence there is no way $r_1$ and $r_2$ can swap places
A: It is a matter of definition of whether you want to revert the order of vector spaces on tensor products or not when going to the complex conjugate vector space, i.e. in physics jargon: from ket-spaces to bra spaces. Different authors use different conventions. 
In particular, in the case of super vector spaces with Grassmann-odd elements, in order to minimize sign factors, different conventions are useful for different tasks. 
A: Remember that by definition of the tensor $$(a_1\otimes b_1)(a_2\otimes b_2)=(a_1a_2)\otimes(b_1b_2),$$
and that $\mathbb C\otimes\mathbb C=\mathbb C$. 
