For me the best way to have two animal companions is tu multiclass 4 level of knight. If you take the horse master feat your mount (wich is exactly like a companion without the spell transfert) will be at last determined by your global level (so if you are druid 16 and knight 4) your mount will be level 20. And then, the boon companion let you have your druid companion at the same level, cause it give you back the 4 levels used on knight. Sorry for my bad english.
ps: use this combination with an human using the alternative racial trait "Eye for Talent: Humans have great intuition for hidden potential. They gain a +2 racial bonus on Sense Motive checks. In addition, when they acquire an animal companion, bonded mount, cohort, or familiar, that creature gains a +2 bonus to one ability score of the character's choice. This racial trait replaces the bonus feat trait." will make your 2 companions much more powerfull (you sacrifice one feat for 2 time +2 on ability score).
Thanks for reading this.