1) pick a pet to merge with
2) while unmerged open hero panel->build
3) click on > on the soulbeast specialization, move the mouse on another core specialization (no druid)
4) click F5 when the animation start click on the core specialization to remove the soulbeast one
now you should be core ranger and merged so you have the passive merge buff + the pet + the beastmastery bonus
you can pick another pet and switch back to soulbeast if you want so youllbe merged with a pet wile also having another pet around