When you have a Kronch Wrangler on the battlefield
Whenever a creature with power 4 or greater enters the battlefield under your control, put a +1/+1 counter on Kronch Wrangler.
and a Bloom Hulk enters the battlefield
When Bloom Hulk enters the battlefield, proliferate.
which ETB effect goes onto the stack first?
Are the triggers simultaneous, so I as an active player can choose to give my Kronch Wrangler a counter, and then proliferate it?
You chose the orders your triggers go on the stack. If you want to give you Kronch Wrangler 2 counters, you would put the proliferate trigger on the stack first, thus putting it on the bottom of the stack, thus causing it to resolve after Kronch Wrangler's trigger.
603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose.
