Di- prefix

Simple ethers derive their name from the two groups attached to the oxygen followed by the word ether, eg, diethyl ether, CH3CH2OCH2CH3. Eor symmetrical ethers the "di" prefix is often omitted. If one group has no simple name, the compound may be named as an alkoxy derivative, eg, 2-ethoxyethanol, CH3CH2OCH2CH2OH. 

In this case both the cation and the anion are complex. To indicate that there are two methykmine ligands in the cation we will use the bis- prefix. [Note that if we used the di- prefix we would have dimethylamine which could be readily construed as one (CH3)2NH ligand instead of two CH3NH2 ligands.] In the anion, the two aqua (water) ligands come alphabetically... 

---